I like randonneuring because the guys in the club know so many more nice roads than I do. I mean it would take a lifetime to figure out all these great routes. Joining the club and going on these rides gives me instant access to decades of this accumulated route knowledge.
It's also nice for the safety aspect. Sure I can go on long rides by myself - and I often do - but I appreciate the safety aspect of having a large group of folks who know where you're going and approximately when you're going to get there. If something catastrophic happens, I have that bailout option of calling the route organizer and getting a ride somewhere - or if something really bad happens, at least I know there will be people looking for me who can find me.
And the people are great! Frankly, I rarely meet people who I am interested in talking to, in daily life. On rando rides, I almost always do! These nuts are just really interesting people, with interesting lives and interesting views on stuff. What better way to get to know someone than a 10 hour bike ride?
Another nice thing, it represents a good balance between just riding along, and a more sporting aspect. I don't always want to flog myself around a track for 45 minutes sweating blood and turning myself inside out, nor do I always want to wander along smelling the flowers. Randonneuring is a good middle ground that suits me very nicely.