I know. What do you expect. I played longer than you.
Underpromotion: Typical signs that you want to underpromote
Underpromotion is where you promote a pawn to a piece other than a Queen. We normally do underpromotion to avoid stalemate, or, may want to promote to a knight to do a knight fork, or possibly checkmate.
Promotion to a bishop is very rare, though sometimes you DO want to underpromote to one. For example, consider the position below:
The best move here is indeed f8=B, since Q and R result in stalemate, and f8=N+ MIGHT be able to checkmate, but it'll take a long time at least, but with f8=B, it's an easy mate in 8:
if 1... Kg8, it will be mate in 7:
I'm pretty sure the vast majority of underpromotions here are for fun i.e. underpromoting unnecessarily.
There is also one other reason an underpromotion is viable.
Promoting to a queen in this case would be bad, as White will probably capture the queen no matter what. Illegal moves are not allowed on this site, so if White accidentally drags the rook to a2, for example, the system would not register the illegal move and will therefore ask White to make another move.
However, Black can promote the pawn to a bishop. This time, if White mouse-slips his rook to a2, the move would be registered into the system, as 42. Ra2 will be a legal move in this instance. If that ever happens, then the newly promoted bishop can escape.
In that situation, an underpromotion to a bishop is much better.
Here is the game above.

You may think of 1. Nf4, resulting in 1... exf4 preventing stalemate, but Black may play 1... Kh8! to make it mate in 9.

the last move should read 11. bxa8=Nch and Black resigned. Note promoting to a Q loses.
to 11...Qxf2#
Underpromotion is where you promote a pawn to a piece other than a Queen. We normally do underpromotion to avoid stalemate, or, may want to promote to a knight to do a knight fork, or possibly checkmate.
Promotion to a bishop is very rare, though sometimes you DO want to underpromote to one. For example, consider the position below:
The best move here is indeed f8=B, since Q and R result in stalemate, and f8=N+ MIGHT be able to checkmate, but it'll take a long time at least, but with f8=B, it's an easy mate in 8:
if 1... Kg8, it will be mate in 7: