As an established idol, is there a reason to be under contract?

I've only been into kpop for a few years and only really follow Monsta X and NCT (no I'm not ok 😭😂) I'm a little confused about the point of a contract. Changkyun ended his Starship contract a few years back, but still works with Monsta X. Ten is ending his with SM but is still going to work with NCT. I'm not sure I understand the point of being under a contract with a label if you can still actively work with "your group" freely. What am I missing?