One of the ways to keep your washes wet and workable for as long as possible is to have the paper saturated - or nearly so.

This can be done by wetting the front side, where your subject is drawn. The problem is that you are left with a surface that can only be worked wet in wet.

Wetting the back of the paper makes it possible to saturate the paper but leaving the painting surface dry. This slows the absortion of water from the front, keeping the washes wetter, while also leaving a dry surface to work.