What Is The Point Of Leveling Trusts?

[Tsukino]

I feel like the most obvious answer is the same as everything else in the game: because it's a thing you can do.

What point is there to collecting 100 different mounts when they (almost) all function identically to the chocobo you get for free? Why would you go back and do old content repeatedly to hear a track of music in your house or to get a completely useless Triple Triad card? Why suffer through 360 FATEs to get some minions, crafting items, and more music? Because they're there, in the game, for you to do.

[Granyala]

I feel like the most obvious answer is the same as everything else in the game: because it's a thing you can do.

Pretty much.
That and it's probably supposed to be a barrier of entry.
If you wouldn't have the lengthy leveling process attached to it, a lot more people would run their expert dungeons via trust because "players are scary and stressful".

The same reasoning is probably behind their absurdly low DPS. So there is still some advantage to running with actual people.