having taught at playgroups that you pay for and from meeting friends with our kids, i can say both have advantages, depending on your requirements
if you read some threads on here and other websites, there are some people out there that have no idea what to do with their little ones, what milestones they should be reaching, what is "normal"(used VERY broadly...i hate the expression, myself), or even have a hard time meeting other mums/dads, etc due to work commitments.
there's NO NEED to go to a pay-for playgroup (my own son has only ever attended if i was teaching), but some parents get reassurance from them.
that said, if you are a confident parent who doesn't feel the need to "keep up with the jones's" then you probably won't get anything out of a playgroup that you couldnt' organise yourself.
another major problem in hk is space. not everyone here lives in a 2000', 4 bedroom flat with facilities. people in much smaller accomodations tend to join more activities to get them out of the house (although i find this ironic... if they took all of the $$$ they spent on all of these activities and applied it to rent, they might be able to afford a bigger place with facilities...)
anyway, my 2 cents...