Some things are better when they're not free.
If Craigslist charged a dollar for every listing, what would happen?
Well, the number of bogus listings and repetitive listings would plummet, making the site far easier to use.
The number of scam artists using the site would go down, because it's more difficult to be anonymous when money changes hands.
The revenue of the site would soar, which means that the people running the site could get (far) richer, or fund digital journalism or change the economy of an emerging nation.
Money creates a sort of friction. In the digital economy, magical things can happen when there is no friction. You can scale to infinity. On the other hand, sometimes you want friction.
If you lead a group that allows anyone to join, for free, your group might be large, but it's not tight, it's not organized to make important change. Commitment slows things down in the short run, but ultimately aligns interests.