The simplest! In the endgame the extra pawn is more important. In general it is useful to exchange pieces when you are material up.

The tactical justification.

21...Nc5 would have been better.

And now two pieces are hanging - the game is over. Black resigned after three more moves.

The second logical thing that springs to mind is preserving your queen for an attack. When your only plan is going for the enemy king, you should logically avoid the exchange of the most dangerous attacker.

A strange decision, as now White will be two pawns down with the queens off. It is well known that opposite-colored bishops with the major pieces on the board favor the attacker. So White should have fearlessly gone for 30.Kg3 Qe5+ 31.Bf4.

White still has compensation due to the passive black pieces, but he loses track further on.

A mistake. White could have played 35.Rh8+ Rg8 36.Rxh5 with compensation.