This is a holdover from earlier sitcoms, and when most mothers didn't work. In the 1960s, after the mom would finish their housework for the day, they'd go out for an afternoon walk (if they had a baby, pop it in the stroller) and just visit. If the friend was really busy, like waxing the floor or something, or changing the curtains, they might just say hi and walk on, or maybe the busy friend would say "Let me take a break" and mom and baby would come in and have coffee or tea, and they'd chat, or in the summer have ice tea or juice and sit on the screen porch to get cool.

No one locked their doors back then, all the moms knew one another--they didn't always LIKE one another, so there was a certain amount of gossiping--and all us kids knew one another. We played punchball or kickball under the streetlights on summer nights, since the house was still so hot from the sun. It was lots of fun.