[Rash Guards vs T-Shirts]

Why would someone use a $ 50 best bjj rash guard of $ 5? Is there any real advantage to a rash guard vs a wick away t-shirt? If so is the the long sleeves one better then the short. I'm guessing it's just a matter of preference but I figured who better to ask then the guys/girls using them.

[Newbie here]

HI all!!
Newbie here been preparing over the month and finally joined. Hope to learn a lot of things from you guys.