Replay value is a very important factor in creating and playing a video game.  You don't want your players to just play it once and throw it in the used games pile never to be played again. Games should be able to be played many times and at least have something new each time.  

A lot of games being recently released are having more and more replay value to their games aside from just multiplayer.  Achievements within the game always make the perfectionists (like myself) work hard at the game to get every achievement and unlockable.  In my opinion though, having a game with multiple endings always has the best replay value.  Anybody remember Chrono Trigger? If I remember right it had about 8 different endings you could get. Different endings are always the most rewarding  to learn more about the story and see what the result is if you did something different earlier in game. Getting an unlockable for beating the game on a harder difficulty is not only a good idea but a great challenge for gamers which includes some well earned bragging rights.  

Some games have a good guy/ bad guy ending which is ok for having different endings but it can probably be assumed how it's going to end.  If you played as the good guy and everyone is saved in the land and replay it to play a bad guy you could probably assume not everyone is going to be saved.  

Multiplayer is a great addition to games to add replay value but isn't necessarily always needed.  It's even better when unlockables from the single player can be used in the multiplayer mode like Rainbow Six Vegas 2 did.  In case you don't want to play online you can play through single player and earn achievements as well as new gear for your character.  It was well planned out and should be considered in other future games.