We need to note that contextually, the dam was blown up at the start of Ukraine's Counter-Offensive against the Russian positions in the south of the country.

Purely mechanically looking at "who benefits" I have the following understanding.

Reasons it could have benefited Ukraine:

1. The flooding destroyed the Russian first lines of defence along the Dnipro as well as their artillery ranging Kherson

2. The flooding created an emergency situation on the ground requiring evacuations and therefore double-tasking Russian forces which were supposed to defend the primary surge toward Tokmak

3. The destruction of the dam makes Russian occupation of the Zaporizhia Nuclear Power Plant more difficult

4. The destruction of the dam makes Russian occupation of the Crimean Peninsula (which gets its water from the dam) more difficult

Reasons it could have benefited Russia:

1. If Ukraine's counter-offensive had included an attack across the Dnipro, this action would have delayed that front, allowing Russian forces to focus on the three other main fronts. (Based on what we know now it appears this was not part of Ukraine's action plan, but that's not to rule it out entirely)