The only one I've gone to in decades is the Pacific Northwest Folklife Festival in Seattle (every Memorial Day weekend).

Originally, I was part of the folkdance community and my main interest in the festival was the participatory dancing, with a fringe benefit being the chance to catch some good stage performances.

As I got involved in volunteering to help, I was also introduced to the participants' areas where lots of people hung out and got into jamming. That became a highlight, both as a jam participant and as a listener.

Over the years, I've become more involved in the music performing and jamming aspects, ranging from being onstage myself to leading songs in song cirlces to joining the jammers on Bluegrass Hill to participating in John Ross's band scrambles.

And of course, seeing old friends and meeting new ones is a great draw too.
NW Folklife is one of the few festivals that's free -- or theoretically can be. Donations are solicited, and you can spend a lot at the food and crafts booths and the CD and instruments areas, but you could take in everything but the food without paying a dime.
Many or most of the acts on stage are the level that you'd pay good $$ to see elsewhere.

While the on-stage performers - from unknowns to big names like Pete Seeger, Ritchie Havens, Utah Phillips, etc. -- are an important part of the festival for me, I doubt I'd travel 360 miles round trip just for that, if the "mates" and jamming and dancing weren't part of the mix. Portland (OR) has all sorts of great free or cheap music you can hear most any week in the summer.   But it's probably the comprehensive experience -- including "letting the festival happen" without overplanning -- that makes this festival an every-year event for me.