I would think that Ukrainian military senior staff know a bit more of what they’re doing than us armchair generals. In terms of the full explanation, I’m sure we’ll get that much later down the line (likely after the war). In terms of the desperation, I guess a bit of that has to be involved. They wouldn’t need to do this if they were winning easily in the east, but that’s where basically all the Russian military is alongside extensive fortifications, minefields, pre-sited artillery, etc.
It’s likely also got a political angle to it too. Undermining the myth of the strong Czar protecting his people, and forcing tough decisions about how to use conscripts, some of which are from wealthy and influential cities (unlike the contract soldiers).