they do it so they can tick the box that says "I tried".
FlySolo brings up a very good point. My W did this. After BD, I tried everything to make her change her mind and not leave. She stayed for 5 months but during this time, she was not trying to fix anything or to work on the relationship. She constantly refused to go to MC and would say things like "I know I could make it work if wanted, but I don't want to try".
Then several months after she left, she started sending me emails stating exactly what FlySolo wrote: "I tried" but it didn't work.
I'm telling you to move on. I think it's wise to be patient until you are certain you want to move on, but don't get your hopes high because she went to 4 MC sessions. You will know if she really wants to work on the MR. She clearly isn't.
Me:49 XW:41, M:18 years, Kids: S18,S14 BD:JULY 2017, W moved out: DEC 2017 Filed for D: APR 2019, D Final: JULY 2019