Most Warriors have one HUGE liability. They cannot hide or cast invis. Due to the new monster spawn strategy used in UO ( one high level MOB clustered by tons of lower level spawn), the need for the Warrior to invis is necesarry for survival AND finesse at hunting. Later, we will have a discussion on how to tailor spawn areas (finesse). For now, let's focus on survival.

So why use an invis potion?
Ever run from a monster, dragging 37563424 behind you?

Ever get yelled at for dragging said spawn on top other players?

Ever WANT to hide, but CAN't?

This potion is for YOU!

Other neat bonuses of the invisibility potion:
You can cast a spell, drink invis, and go hiding/stealth with the spell target rdy

If you get dismounted, you can hide and cast your ethereal mount if you have one

Most chars dont have means to discover you, remember that.