I can actually understand it. It has tons of issues, but it was the last "big" 3D sonic game.
It felt like Adventure 3, while all the games after that were a lot smaller or had weird gimmicks, or were 2D games (not that it's a problem, I'm just talking about why it feels different).
Sonic Generations was great but was more of a compilation / remake thing, and had a lot of 2D too, and all the other games since that were similar, there hasn't been a full big 3D game since 06.
I think Sonic 06 could have easily been a decent game with some correctives and improvements. I enjoyed it as a kid but I'm not sure if I could go through the whole thing again in its current state, the loadings alone are too much.