It probably isn't in every case but my wife is passionate about it and treats it as a full time job. She takes our daughters out to social groups, meetups, farms, gymnastics, swimming.. every day. There's nothing wrong with daycare (my eldest went half-time for a year for the social opportunities and because it's free in the UK ;-)) but a full-time parent can provide even more variety and social opportunities with the right resources and the right community around them.

One unexpected upside is my wife spends so much time in the community it has really extended our circle of friends. The biggest downside, however, is I still get to enjoy half the chores because my wife is rarely home in the daytime(!) ;-)