B+N worth R + how many pawns? Purely by piece counting, B+N is worth R + 1P. But I've seen lots of time when the exchange favours the side who holds B+N. In one variation of Scotch, white exchanges R+2P for B+N and is still slight in favour.

What are the ideas surrounding this type of exchange?
Only a general answer can be given for this:

Generally the exchange of R+P (sometimes an extra pawn) for a Bishop and Knight favors the player who has the Bishop and Knight afterwards due to the simple fact that the Minor Pieces are much stronger during the middlegame than the Rook. You have two attacking pieces vs. one attacking piece. The side with the Bishop and Knight will have better attacking chances as they will have more open lines (diagonals are much more open than files in the middlegame) and good squares for the Knight. The Rooks will be blocked by the pawns and unable to contribute much to the game.

Of course, it is very position dependent. In various variations of the Sicilian Defense, Black will exchange a Rook for a Knight on c3, sacrificing the exchange. This is extremely important for Black to do as it creates weaknesses and increases the strength of their dark squared Bishop.

In addition to this, there are many attacks that use the Rooks in the middlegame by opening up the h-file. It's very position dependent as to which group of pieces is more important or not.

As a general rule of thumb though, a Bishop + Knight is considered more important and stronger than a Rook + Pawn.