Leveling trusts allows you to use Trusts to replay the dungeons from level 73-80 with a Trust (With level 71 always being available without leveling them up).

This means, you can level alt jobs via Trusts without having to rely on DF, which can be useful for leveling DPS jobs with poor queue timers or if you're playing at a non-prime time.

It also allows you to farm gear from said dungeons for use or glam (Without having to roll against anyone else)

I think the level 80 dungeons also give you Tomestones too (I know they do when you do them the first time)

Beyond that... Not a ton of value for doing so. But that's not a particularly new phenomina for FFXIV content