Sometimes you will have a pinning opportunity but the value of the piece you can pin is too low to be exciting—and maybe even too low to be workable. Again, exchanges may be used to upgrade the target to something better.
In this first case, a simple scan of the enemy king’s lines turns up a pawn on e5 ready to be pinned. You might imagine Re1, but notice a fatal problem with it: the pinned pawn can step forward to e4. Now it has protection, plus it attacks White’s knight, plus it has unmasked an attack by Black’s bishop at g7 against White’s loose pawn on b2. (When a player has bishops arranged like Black does here, be very careful to consider the consequences of pawn moves in the center that may open paths for them.)
But don't give up. Where the enemy has anything on a pinnable square you have an important tactical opportunity: perhaps the lowly pawn can be exchanged for a more valuable and vulnerable target. Here White invites the substitution with Nxe5. If Black replies BxN, then instead of a pawn on e5 we have a bishop. White can pin it with his rook or queen; he chooses the cheaper piece and plays Re1. Black has no effective defense. He can play his queen to g7, but then White throws another attacker at the target with f2-f4. The bishop must be lost, resulting in the gain of a pawn and a better position for White (he ends up with a strong center). Black probably is better off skipping Qg7 and instead playing his king to f7. This makes room for him to reply to White’s Rxe5 with Re8, which prevents White’s rook from dominating the file opened by its capture of the pawn.