Enlighten me please as to why it's necessary to remove the brake pads when doing a fluid bleed?

My experience with auto disc brakes has always been that bleeding the system (of possible air entrapment or for a replacement of aged or contaminated fluid) is done with pads in place.

Any potential for pads to be contaminated by leaked fluid is supposed to be circumvented by placing a hose over the end of the bleeder ports to redirect escaping fluid into a proper reservoir.

Is this not possible with popular bicycle hydraulic brake systems?

(Being new to hydraulic disc brakes on bicycles I fully expect to have to get up to speed with proper maintenance sooner or later. I'm not at all the type to leave what ought to be routine maintenance to my LBS staffers. There's only one LBS in town, any others are a fair drive distant, no co-op closer than a two hour, one-way drive.)