Most people wait until they have a steady source of traffic, especially returning ones.
Then they open the forum and ask people they know (or they do it themselves on multiple accounts) to post and try to spice things up.
Forums are annoying to set up. My newest website gets ~50 uniques per day (last month’s average) as well as returning visitors, but only 15 people signed up on the forums so far and out of those, only 5 actually post (3 of which are myself).
I do think the advantages outweight the disadvantages in the long run.