Well, in WoW, you need a lot of gold to buy things like very fast mounts, land and air mounts. It takes months of farming and doing quests, or you can have a bot play for you 8 hours a day while you sleep.
For the people who can only play 2 or 3 hours a day at most, having your computer play for an additional 8 hours would be a huge help. Instead of farming, I spent most of my time in auction halls trying to buy low and sell high, but that really cut into the couple hours of play time I could put in every day.
WoW isn't a friendly game to the casual player. You're either addicted and play 10 hours a day, or you don't get far very quickly.
I'll just use fishing as an example.
30 second average to catch a fish
0-50 points in fishing, you get one per fish caught.
50-125 you get one per three fish caught
125 -250 you get one per five fish caught
250 - 375 you get one per ten fish caught
50 x 30 seconds = 25 minutes
75 x 30 sec x 3 avg = 112.5
75 x 30 sec x 5 avg = 187.5
125 x 30 sec x 10 avg = 625
950 minutes / 60 = 15.8 hours of fishing to level it.
So, on top of the regular playing that you do, you'd need to fish for 16 solid hours (not counting breaks or sleeping) in order to get it to max level... or what used to be when I was playing.
So, you can spend 2 nights of letting a bot fish while you sleep, or take 8 solid days of your play time leveling (grinding) something so you can get a higher level at it.
I'm sure my math is off, but you get the point.