Firstly a 2nd ref could bring a different result (deal or remain) than the current likelihood of no deal. If no deal Brexit does happen there'll be financial repercussions in both the UK and the EU. Then the blame game will begin. It's important for the EU and for the EU leaders' domestic agendas to have been seen doing everything they can to prevent it, while obviously respecting UK sovereignty and protecting EU interests along the way.
The UK has opened the exit door and is currently standing at the exit, dithering. But they don't want to be seen as basically shoving the UK out and bolting the door. That won't play well with their electorate and won't do the EU reputation any good. They'll appear as vindictive, as opposed to the mature, responsible adults. The longer this plays out, the more harmonious they appear in the media in contrast to the chaos of UK politics, the more it serves as a warning for other members of the very high self-inflicted cost of exiting the Union.