I re-play games, re-read books, re-watch movies, and listen to songs I've heard tens, or hundreds of times before. Sometimes I know exactly what I want to experience, and that's only really possible if it is something you have done before. Life is not a race. Noone is going to count how many unique thigns I managed to experience before I die, so I see nothing wrong in spending time doing things I know I will enjoy, over again. Life is too short to let others tell you how best to spend your time.
Anyway, there are three reasons I can think of that lead to me replaying adventure games:
1. A feeling of having missed out on something while playing. This can be due to a branching storyline, a complex story that didn't sink in completely, or a twist at the end that I feel I missed out on the setup for. I'll usually play the game again while it's still fresh in memory if this is the case. Knowing exactly what I need to do next, means I can try everything else first and not miss out on anything.
2. A great story. The most common reason I'll replay a game. This is exactly like watching a movie, or reading a book again for me. Discovering something I haven't done before, or nuances to the story would be a bonus, but mostly I'm just after a familiar and pleasant experience with enough interactivity that I feel like I'm actually doing something. There's no pattern to when I'll play something again for the story. It's "whenever I feel like it", but there would normally be long enough between playthroughs that I'll have to stop and think about what to do next, even if the solutions are lodged in my memory somewhere.
3. Nostalgia. Sometimes I recall a game I enjoyed immensely in the past. Playing them again usually won't recreate the experience, but I'll remember why I enjoyed them so much. Triggering good memories, can't be a bad thing? I often play a game for nostalgic reasons after I get disappointed with a new game, and get reminded of something I played a long, long time ago that, according to my memory, did the same thing better. ;p
