Support. I'm lugging around some p2p items everywhere I go right now (I go between f2p and p2p). It really helps to be able to go to f2p without having to worry about storing f2p items or accidentally withdrawing p2p items.
By the way if you're carrying your coin stack around you can "store" it in the GE. For example, if you have 1m gp you can just put in a buy order for 1m runite bars at 1gp a piece. That's what I'm doing currently.