I've addressed this elsewhere, but, to keep it in one place:
Going through your stash and figuring out what you want to use, where your tastes have changed and you now know you won't ever want to use, and what you didn't get around to using before it became otherwise unusable, and then paying attention to and remembering these things, can help you keep the stash from growing so much in the future
Having your hobby stash be more focused can help you be more likely to use the things, because it's not as overwhelming
Actively using the things can help you see that, quite possibly, it doesn't take as much as you thought to make the things you want to make, and your existing stash--even after you've started using it, given a lot of the unwanted stuff away, and gotten rid of the ruined stuff--will already be able to provide materials for years' worth of projects
And it can help you be more selective when acquiring more--for example, I am now a lot better at recognizing that, yes, I might be able to see the potential in something, but, if it's not necessarily the potential for something I actually want to make, so I am more able to pass on it
It can also help you identify gaps--there might be something you've been making do with that works well enough, but is annoying or painful or otherwise unpleasant to use, so you are far more likely to just not do it at all, and there might be something that would solve that. This is not the same as the thinking that says "If I buy this next New Thing, then I'll really start the hobby!"—this is about already being engaged with the hobby and analyzing what could help with what you're already doing. (This applies to not only tools, but also things like taking the time to get a sewing pattern adjusted to fit the way you want, or re-arranging storage so it works better for you, or making a point of de-linting and oiling and changing the needle on your sewing machine on a regular basis)
Notice I used the auxiliary verb "can" in the paragraphs up there—nothing is universal, and I acknowledge that. What works for me will not necessarily work for you.
For me, personally, though: I gave away so much (I have crafty friends and access to a craft thrift store and that helps), threw away a lot (so much cardboard), used a large amount (including most of what I brought in after analyzing the things I let go), and my craft stash is still enormous.
It is so easy to lack perspective on our possessions, especially when all we do with them is put them in storage with Dreams of Someday, y'know?