It's hurting Russian speakers.
It's hurting Russian defensive lines.
It hurts Crimea's water supply.
Why would Russia do this?
Because Russian-Ukrainians and 1/5 of a person (at best) when they don't serve a propaganda purpose. Hurt them, whatever, there's a fly in the room!
Screwing with a Ukr offensive is the purpose of defensive lines; the *flooded* Dnipro is a defensive line on steroids.
It screws with Ukr water supplies--drinking, irrigation, whatever. The Ukr russophones? "1/5 of a person" ring a bell?
It hurts Crimea's water supply? Meh. All the news always and only says it was the Ukr that effed over Crimea's water supply. Rah-Rah Rahssiya! Ewww Ewww Ukraine! All the news that fits sees print.
Or maybe somebody just screwed up on the Russian side. "Oops. *They* planted explosives? And *we* planted explosives? And others were planted last June? And nobody talked to each other? So we had *that* much explosive? Didn't anybody talk to each other ... Think it through ... Hey, you got my 100 grams? I want a loan for the next 2 weeks. ... Pay it back? Harvest my organs, I'm dead."
I haven't heard any idea about when the former reservoir is now just another bit of the Dnipro that deserves a front line constructed. I'd seriously love to find that the Ukr forces have managed to establish defensible territory on the left-hand side of the Dnipro on the former bottom of that reservoir when the rusnya forces decided it was dry enough. (But I'd hate to be the first in the muck. Reservoir bottoms are like unkempt and neglected human bottoms--who knows what crap accumulates there, and it's rash to show up too soon.)