I know it's a small thing that shouldn't bother me, but...The cats in Seasons 10 and 11 are Spike and Xander's, not Spike and Buffy's.
I'm a Spuffy shipper, but the cats aren't theirs. I see this a lot on Twitter. Spuffy fans who haven't read the comics got this misconception that the cats are Buffy and Spike's, but really they are Spike and Xander's, and this is important to acknowledge. Buffy pets the cats once in a while and is cool with the cats cuddling with Spike in their bed, but she never cares for the cats, barely interacts with them, and would likely not view them as hers in any way (like with Miss Kitty Fantastico - that was Tara and Willow's cat).
The cats being Spike and Xander's thing is so important, especially for Spike's character growth. In the TV series, we see that Spike is constantly living his life and creating himself and his persona after whatever woman is in his life and takes center stage in his eyes, whether that is with his familial love for his mother or the more romance focused love and relationships that he has with characters like Drusilla and Buffy. Harmony is a female character that Spike ends up with during very transitional times for himself:
Season 4 & Early Season 5 - when his character is very lost and trying to find a place now with the chip in his head.
Angel Season 5 - the brief fling that we see him have with Harmony after he comes back to a solid state of being.
Basically, Spike is a male character that has a very long and storied history with women and especially Buffy. So much of Spike's character is tied to Buffy and to the passion of lust and romance. A key element of TV show Spike's character is him being Love's Bitch. A lot of people love his speech in Season 3's Lovers Walk, and I do understand and get why, but I also feel like his perspective on romance and love is actually rather immature and shallow - a juvenile take that speaks to his issues with codependency.
We see Buffy and Spike move into a healthier direction in Season 7 of the TV show, but Spike's (and Buffy's) codependency is still a problem that needs to be worked through and overcome. Over the course of Angel Season 5 and the comics, we see Spike finally reaching a place where his sense of self and identity isn't completely tied to the women in his life.
Over the course of the TV show we also see that Spike doesn't seem to bond easily with other men (he has Clem and the other demon guys he plays kitten poker with, but that's not quite the same and doesn't really apply to the other human men in the show like Xander or Giles). But, by the end of Season 10 we see Spike having a stronger bond with both Xander and Giles (I'm not even going to bring up Angel here, because that is a whole different post). This is a long journey that Spike has been on since Season 4, when we first see him living with and co-existing with the male characters like Giles and Xander. The Xander and Spike roommate situation is very important, because we see an evolution of it that reflects the growth of both Spike and Xander over the course of the series as a whole.
In Season 4, Xander and Spike are in very transitional phases of their lives and Spike living with Xander is forced upon them both. The place they are living in is a temporary and transitional space that doesn't belong to either of them (Xander's parents basement). There is animosity and resentment. It's funny, but also awkward, and the bad blood is still strong.
In Season 7, we have Xander in more of a position of power over Spike, not only because Spike is staying in a place that Xander pays for, but also because Spike is in a bad place. He is still not fully in his right mind because of The First and his newly acquired soul, and Spike is still adjusting to having a soul. He's still very unsure of himself and his place. There is some bad blood still, but it is more subdued and both Spike and Xander keep more to themselves rather than antagonizing each other. We've seen Xander understanding Spike a bit more over the years, but with the fall out from Season 6, Spike wouldn't be considered a friend or part of his circle still. Spike is a necessary ally in a fight that involves something bigger than all of them. Still, we see Xander not automatically jumping to the worse conclusion with the whole Sleeper situation with Spike and there has been obvious growth with both Xander and Spike as individual characters and as a pair.
In Season 10, Spike has lived his life away from the Scoobies for a number of years and Xander has moved on from Buffy. They are both older and more mature as individual characters, and they understand themselves better. Spike is more self-aware of his flaws when it comes to himself and women and his own lack of self-worth, while Xander is actively going through therapy and working on himself. They both get an apartment together and are sharing the costs together. Spike ends up in a slightly better state (at least mentally speaking) than Xander in Season 10, but there isn't the same power imbalance and the bad blood is mostly gone. They still have their little disagreements, but by Season 10 Spike is considered part of the Scoobies and Xander would likely view him as a friend.
And this is where the cats come in. They are a gift from Spike to Xander, specifically, and they are dependent beings that Spike and Xander raise together. In Season 11 we even see them talking about who is watching the cats like the cat dads that they are together.
The cats are theirs and specific to their bond. The cats bring Spike and Xander closer as friends and, though I don't have any love for Season 12, the cats are likely a big part of what make it so that Spike is a welcomed guest in Xander's own home. Even after Spike and Buffy are no longer dating (on a break, however you want to view their situation in Season 12, lol). Spike no longer needs a direct connection to Buffy in order to be a part of the group, because in the comics we see him building and having friendships and bonds outside of Buffy and Dawn.
He isn't the a co-dependent Love's Bitch anymore. He's still Spike and he still loves deeply and passionately. He still cares about those around him and wears his heart on his sleeve, but he also has a better and more rounded understanding of himself and is able to function and exist outside of a romantic relationship (or outside of having a crush/a rebound relationship/etc.). The reason for that is largely due to things like the Spander Cats.
I love Spike, I love Buffy, I love Spuffy, and I love the way their relationship grows and evolves and changes over time...But not everything involving Spike or Buffy is specifically tied to or about Spuffy.