I find the replies in this thread to be truly lacking.
I looked this up just now because I was asking "what the hell is the useless Trust system for? It's Duty Support with extra steps."
Initially I figured that once you leveled up your Trusts more, then you could run them through content unsynced, so I could farm some glams or cards faster than pure solo unsynced. But no, they get synced.
So... you can choose who goes? Who cares? That doesn't functionally change anything. The entire trust system could be deleted and its miniscule "feature" list added to the Duty Support system, or thrown in the trash. Why clutter up this already menu-overloaded game with two identical systems except one is a noob trap that's just a bunch of work for basically nothing?
The devs should be trimming fat, not adding it.
If you absolutely HAVE to have it for the people who are so deluded they consider it actual content, have it be a sub-tab of Duty Support. Don't give it a dedicate menu option. Good lord, show this to anyone who doesn't already know what it is and how it works, then explain how it is presented in-game and how it functions, and enjoy their derisive laughter.
"So yeah, you use both to go through dungeons without other players, but in this one, you can choose who to go with! Well, no, you can't make unusual party compositions, so it functionally plays near identically except Alisaie likes to steal the LB. Okay yeah, you have to run like a million dungeons to level them up to even get them into the content while in the other one they're already leveled and ready to go. Well, no, you don't level ALL of them at once, just the ones in the party. Why are you laughing? This is important! I can take characters who weren't canonically there! Hey! Come back!"