I usually replay my video games for the following reasons.
1. To relive my childhood.
2. Because I already completed all my new games.
3. To make a walkthrough with a friend or two.
4. Want to relive a certain moment within the games.
5. Because my old consoles look dusty and look sad that I'm not playing their games that defined my childhood and Nintendo's awesomeness.
I love replaying because you get to re-experience the story and remember things that you forgot and remember why you loved the game in the first place. I replay games all the time What's the use of getting a game if you're only going to play it once. It wouldn't be worth it and the game couldn't have been that good if you only played it once and never looked back.
If the game had trouble keeping my attention, I won't most likely replay it. Games like Final Fantasy, Elder Scrolls, Fallout and RPGs in general have high replay value because of the huge amount of content and enemies in the game. Games like Zelda pride themselves on great stories and items. Games like Mario, Sonic and Crash Bandicoot need fun levels that aren't cheep to have high replay value.