Something I both love and hate about TMNT 03 is the fact that it forces you to pay attention to the space arcs. This might be an unpopular opinion, but my least favorite parts of both the 03 and 12 shows are the space arcs. I know you can't really have ninja turtles without some sort of alien fuckery, but I don't particularly care about aliens and I really don't care about the turtles going to space to meet the aliens, that's just a personal preference.
But the alien arcs in 03 aren't something you can ignore or write out because they're. So damn important. They influence how the whole show goes. Turtles In Space is important because it introduces the Fugitoid, and him being brought back to Earth with the turtles at the end of the arc is what eventually leads to the Triceraton invasion in season 3, which introduces Bishop. The aftermath of the invasion is what allows Shredder to gather enough alien tech to build his space ship, and then his plan to leave Earth fails partially because of Bishop's interference, which is a factor that leads to his eventual defeat. And then THAT leads to Karai taking on the mantle as the new Shredder, which then leads to the Foot Mystics using Bishop to free themselves, which leads to the Ninja Tribunal at the end of season 4. I haven't watched season 5 yet, but I know what it's about and I know that it wouldn't have happened the way it did if the turtles hadn't gone to space all the way back in seasons 1 and 2.
And that's good because it's a story that strings itself together very well and, again, it forces you to actually pay attention to what you're being told, because what SEEMS like disconnected nonsense actually slots together later to create a full storyline, but my god is it frustrating for me specifically as someone who doesn't like it when the turtles go to space because THE TURTLES GOING TO SPACE IS VITAL TO THE WHOLE DAMN PLOT-