Why? Convenience. Easy to roll out the door from home. I ride at my own pace. I choose the routes I enjoy. I ride on my timetable. There is no competition and I can ride for my own enjoyment. It is therapeutic (hip injury) to ride alone. I do not have to explain why I carry four water bottles?
Well sometimes I do anyway when asked going up a long climb.
Why not? Companionship. Learn from others' cycling experiences. Groups can be fun and safer. Shared experiences. Mutual competitive accountability if training for an event. Encouragement. Coaching.