besides all the electrical conditions where you have to have the apu running continousely, there are also some circumstances you have to use apu bleed air as a backup during take off. for instance if you have only one center hydraulic primary pump working. if you look it up in the mel there is quite a lot of switching to do before take off!
in case of engine failure during take off one engine could not supply enough beed air for the hydraulic demand pumps to get the gear up at normal speed.