Because it helps with chlorine, chloramine, ammonia, nitrite, nitrate and heavy metals. Unless you're 100% sure none of these things are in your water what does adding it hurt?
From Seachem...
"Prime removes chlorine, chloramine and ammonia. Prime converts ammonia into a safe, non-toxic form that is readily removed by the tank's biofilter. Prime detoxifies nitrite and nitrate, allowing the biofilter to more efficiently remove them. It will also detoxify any heavy metals found in the tap water at typical concentration levels. Prime also promotes the production and regeneration of the natural slime coat"