I think there is the life admin aspect. Making phone calls to British Gas, etc. waiting in for repair technicians. Being there when the kids get home - some days. Also cleaning and organising the house. Arranging Christmas. It stresses some people when they feel they have too many tasks to do. I guess that the above is only relevant if you can afford to only work part-time.
I've met women who express that they think their husband should be the breadwinner and what they themselves provide should be just for financial extras. They do most of the housework and child-related stuff, and feel that that is 'work', and they don't have time for a full-time job. I get the impression that a kind of principle is at stake. With some women I've known, it's as if they feel that once they showed that they had time for full-time employment, that their partner would take it for granted that they would keep on doing it. Perhaps, once their partner, if self-employed, starting doing her part of the child duties and housework, he might find it was easy and that she had the cushier number.
Also, working part time must be almost, but not quite, like being a supply teacher, instead of a permanent teacher, on the books. You don't get sucked into the same level and calibre of office politics and certain stressful imperatives which put a strain on full-time workers. You might feel more like you're floating on the top of the ocean of responsibility.
Maybe it's a status thing, only working part time. Maybe it says: ' we are rich. I'm a certain sort of person'. Perhaps there is a clique type of thing going on, where part time has a certain cache, and belongs to a certain group identity.
Working full time is good, I think, if the kids are teenagers, and are happy about it and manage a lot of their own affairs.