I wear a Nike DriFit under my jersey. I have found that it pulls sweat away faster than an jersey alone. Why, you say? I think because the DriFit is so tight it can absorb the sweat easier and transfer it to the jersey which evaporates.

A jersey alone is not sufficient to do this because it is much looser than a DriFit or UnderArmor.

Proof? At the end of my rides my DriFit is almost dry and my jersey is slightly damp. Also I don't have that 'sweat dripping down my chest and back feeling' throughout a ride.

Also is is just a touch extra protection on my upper body if I crash. There is one more layer of lycra/material to tear before the pavement gets to my skin. I know its minimal, but I would rather have that than just a jersey.

Something like this is purely preferential - just like chamois padding or saddle choice.