Regular porn does the job 90% of the time, but onlyfans provides you the option to watch some exclusive content others don't have access to. The exclusivity aspect is what makes it so appealing.
The appeal of only fans is the interaction. You can say the things to a girl you'd like to say on Tinder and she'll respond positively rather than unmatch you.
In my experience, it's not worth it though. As with most things female: too much effort, not enough reward.