I use a CombiPlan tank and prewet (not soak!) films mainly to reduce the chance of air bubbles forming during development. As such, I drain the water after 10-20 s (just like the previous poster). This 'prevention of bubbles' is more of a belief than a fact, as I don't have proof that it's necessary.

The prewetting also eliminates the indigo/grey color so that the developer stays clear (I reuse it).