Bath Exhaust Fan usage?

http://www.hgtvpro.com/hpro/ex_edg_homeowners/article/0,3140,HPRO_28918_5299448,00.html

Article linked says the proper way to use a bath fan is to turn it on 15 minutes before you shower and turn it off 15 minutes after.

I can't for the life of me figure out the necessity of turning on a fan before you start to add humidity? Anyone?

Only thing I can think of is that it takes several mins for the fan to get the aflow going thruout the room?

Maybe that is about lowering the humidity as much as possible beforehand by removing any accumulation of condensation related to any of the fixtures since the last time the fan was run.