Quoting Irving Chernev:
A brilliant sacrifice which must be accepted. Refusing the Pawn means that Black could never free himself by … d5. It would also enable White to play 16 ♘c3 next move (attacking the Queen) and thus gain time for 17 ♘d5, establishing a strongly supported outpost.
Chernev, Irving. The Most Instructive Games of Chess Ever Played (p. 37).
I have read this multiple times in this book, but I don't quite grasp why an outpost guarded by two pawns is considered stronger than an outpost guarded by only one?