BA was one of the first airlines to give a mobile boarding pass option. Now in the days before that (and yes, I was around pre BA.com) there was a certain amount of nervousness about giving someone access to more than one boarding pass, whereas these days the kiosks will cheerfully spew out 72 copies if you really wanted to do that. But the point was you were only allowed one shot at getting the boarding pass, so much so that one tended to select a download option just in case the printer got stuck - you could not readily get a second boarding pass, except going to a desk. So the whole show was originally built around a single go at getting this boarding pass. Life has moved on and so other airlines are much more flexible in this space, indeed Ryanair just treats the idea of reselecting new seats after boarding pass download as another opportunity to make a few more Euros.

Obviously if you are not 101% sure that you have the seat you want, you can just leave the boarding pass download side until the last moment, and that's been the advice in this forum for as long as I can remember.