Russians blew that dam, or rather its predecessor, in 1941 to slow the German invasion. The dam was repaired, then in 1943 the Germans blew it to slow the Russian offensive. Lot of war and long memories in that location, blowing the dam was probably always in the Russia plan.
https://www.rferl.org/a/ukraine-dam-destruction-flood-1941-world-war-ii-german-invasion/32448719.html
It’s not entirely clear to me how lasting the defensive effect is. The Ukranians didn’t lose any troops or equipment, the floodwaters are receding, in a couple weeks the ground will be dry and if the Ukranians had planned to cross downstream of the dam, they can do it. Crossing upstream will be easier as the reservoir will be much smaller. And from reports, it seems like the Ukranians may be attacking in a different location entirely. I do wish the West had gotten them aircraft.