1) Adult Socialising
Groups are a chance for stay at home parents to get to have adult interaction in daylight hours. A time to gossip and compare notes with other grown ups over a cup of coffee or tea. This may lead to new friendships as time goes on.
For some people this maybe the only time they get out of the house during the week. As such they are vital for the mental well being of parents.
2) Children's Socialising
A chance for children to learn about playing with other children from outside their family and normal social circle. Gives them a chance to mix with a wider group of children.
They can also build on skills that they are learning elsewhere such as sharing which is more important in a large group.
3) New Experiences for Children
Often at groups there are toys and activities on offer that aren't available to them elsewhere. Either because there are a finite number of toys you can keep at home or due to economies of scale different crafts are available each week. A boon for mums that can't cope with messy craft at home.
At some groups they are visited by other bodies such as Toy Libraries or Children's Centres who bring in special games and activities on an occasional basis.
4) Fun together
Away from the pressures of home and distractions of housework, TV, phone calls etc. playgroups are a chance for adults and children to have fun together. Whether this is trying a new toy, getting messy on the craft table or as part of the sing-a-long it can be so much more relaxing away from home.
Not so good
In some groups there are still very few dad's and granddads so men can feel like odd ones out. This is changing as there are more SAHD's and hopefully more will come and join in the fun the mums have been having for years.