I'm not sure I understand what they are anymore. I'm pretty sure before Endwalker, Trusts had to be leveled in order to be used in dungeons higher level than they can do but now you can just do them whenever using Scenario mode instead of Avatar mode.

Other than being a completionist, is there a purpose to level your Trust partners? Does your loot improve if you go in Avatar mode instead of Scenario? Does Thancred stop saying "MY TURN" and move the stack marker in danger? lol jk

To quote Seinfeld, What's the deal with leveling trusts?