If Ukraine suddenly becomes cash rich, then suppliers could start upcharging them for supplies. Not good for anyone except defense contractors.
If the EU does this gradually, then they could use this as a bargaining chip against Russia. Think "we are going to liquidate $1b worth of your assets and transfer the proceeds to Ukraine each day until you withdraw from Ukraine and agree to respect their 1991 boundaries" kind of a deal. If Russian oligarchs know that there's a possibility to get their fortunes back if Russia ends the invasion, they might keep up the pressure on Putin to call it off.
SCarolinaSoccerNut t1_iyedf8y wrote
Reply to comment by [deleted] in The EU is looking at seizing $330 billion in frozen Russian assets and investing them — with any profits going to Ukraine by KeenlyFirst