Why not set them manually?

Automatic channel selection is fine for a simple setup with one WAP, but multiple WAP's can interfere with each other quite badly, even when they are out of range of each other, unless you set the channels carefully.

Ideally no single device should be in range of any WAPs whose channels overlap. Something that cannot be guaranteed unless the channels are manually configured.

the auto channel select algorithms seem to leave much to be desired, I frequently see BT home hubs on channel 5 or 7 thereby overlapping two of the non-overlapping channels.