Pokrovsk is in the news, but Russia is cutting off Kupiansk from the north and west at the same time. It looks like one big cauldron forming around Kramatorsk and Sloviansk.
Kramatorsk and Sloviansk are the Ukrainian stronghold in the Donbas and most definitely the last cities to fall if Russia takes the whole region. Since we all agree that taking the whole Donbas is the absolute minimum goal Putin wants to achieve, the timing seems to line up pretty well with the start of peace negotiations.
If the dominos in the Donbas fall very quickly from here on out, Putin can use the credible threat of further gains as a bargaining chip in the negotiations.
If the push on Kramatorsk/Sloviansk goes badly, he can demand Ukrainian withdrawal as a key concession.
If Russia is barely able to achieve its minimum objective in time, they can frame a settlement enshrining those new lines as "mission accomplished, this was everything we ever wanted".