Why do people have more than 1 account?

I have 3.

One I use at work.

One personal I use for important things like banking etc.

One I use for junk e-mail.

There may be many reasons.Few of them I can state are
1.They want to keep things seperately
2.They have run out of the storage spaces
3.They want to have ID on all the websites
4.They want to see & compare the different IDs etc.