Some of each.
It depends what your friends are doing.
There are certainly people who come to Whitby for the sessions.I know there is a group that camp together each year and spend all their time in a pub session.
Some who spend most of their time at sessions but pay a fiver for a workshop most days (and are careful to allow time to get to the workshops - Whitby is spread through the town and both sides of the cliff, and if you have a season ticket you might dash out of one event and make a ten minute sprint to get to the next not too late).

I started going to Whitby as a ceilidh dancer - it expanded my horizons in all sorts of ways. Dancing among experts in Playford, American, Irish sets, tasting the workshops in Sword and clog. Then going to the Harmony singing, and Join the Band. Then we had a break, but went back after we'd started going to tunes sessions - Whitby has sessions for every level of player (including a 'Not quite ready for a Session', and George Garside's Steady pace session). We tend not to go to the singarounds and shanties at Whitby - I think they are hard to get a good seat in if you arrive in a dash.

So yes, people do come to Whitby with special interests and find others who take those interests to a higher level - we dance dances that I would never dream of calling at home. But also try other things - I was early in the pub for a session and found myself with a set of bagpipes under my arm.

The halfway house between week-season (paid in advance is cheapest), and pay-as-you-go, is to look at the programme and work out if there is any day when you want to go to several things. Day/weekend tickets can be good value.

With a week season I can do as I please - start the evening in the Ceilidh, when I've had enough or it gets crowded pop downstairs to the dancers dance, look into the latenight ceilidh even if I don't stay long, or go over the road to catch one act in a late night concert. And not worry that I'm wasting my money on 4 acts I don't want to hear, or even sleep through.
One year one of our group didn't have a season ticket - she found it expensive to keep up with our wanderings.
I always buy my next ticket at the extra-early-price, before the festival ends.