This also defends against White's threatened Qe4, as Alekhine remarked

Vacating the a2-square - and not only for the bishop . . .

15...Nxe3 16.fxe3 Bf6 would have maintained the balance.

This loses right away. Much more interesting was 2 0...cS!?

22.Qe2 would have won simply. Alekhine/Kotov do not mention this possibility. After the text move White had to win the game all over again - which he did.

'Surely the white queen doesn't lack space here?' you'll probably say. Yet Rozentalis decided that he could not break through on the kingside and that he had to open the position up somehow. He managed to come up with a really fantastic concept.

The queen is looking for activity on the queenside to create a second front!

A familiar sort of creeping move.

Of course, it is possible for a simple exchange of queens, solving a space problem, to be the goal of a creeping queen maneuver. Here there is more to it, as the b-file is now opened up for the white rooks.
In the current case, Rozentalis even considered the exchange on a3 to be the decisive mistake. He won the endgame in equally great fashion.