I like the answer in #3.
There are at least a couple of scenarios where I think it's still important, even at lower levels though.
If you advance pawns around your king so that they are protecting a single colour, then it's often important to keep the bishop of that colour for defence. Otherwise they can be key squares for your opponents attacking pieces.
Second is when a pawn is backwards i.e. behind the adjacent pawns and can't be safely advanced. If you have or can create an open file against this you can put serious pressure on the pawn. Even if you can't win it you may be able to tie up your opponents pawns in defence.
I have no concept of scanning for weak squares.
Can someone point me in the right direction o getting started?