Welcome back.
UO Assist is used for making more complex and efficient macros than you can write in UO.
Quicker and more-efficient arm/disarm and dress/undress than you can get through UO client itself.
The UO client can have bandage self keys now, but UO Assist's bandage self macros (which considerably predate those in the UO client) appear to work better and comes with bandage timers.
Drink potion macros.
Mount/dismount.
Plenty of other things.
You're also supporting Tugsoft which has offered support for UO Assist in a manner that has surpassed all reasonable expectations.
If you're playing on the new Enhanced Client, not only will you not need UO Assist but I don't think UO Assist works with it.
-Galen's player