It's so easy to lose track of time when playing World of Warcraft. You start out grinding this reputation, leveling that alt, then get invited to go run an instance, then come back and check the auction, then inspect some people standing around in their pretty epics... and suddenly discover that it's 1 in the morning and you've spent many more hours playing this game than you had planned.
Then again, for some people, limiting themselves isn't at all the problem -- it's limiting their kids they worry about! Their children agree to only play for 1 hour, but then 2 or 3 hours go by and they're still in there at the computer, saying, "But Daaaaad, I forgot!"
If either you or your loved one needs a gentle reminder about how much time has gone by while adventuring in Azeroth, WoWTimer may be the addon for you.