Why do candidates stay in the race when they have no chance to win?

[hotpprs]

I started replying about this on another topic, but I think it deserves a full topic.
Why do candidates stay in the race when they have no chance to win?
I replied something to this effect about Newt, that he has no chance to win and should drop out. But thinking more about this later on, that's not necessarily true.
If Santorum and Romney have no major health issues, don't drop dead, have no unknown scandals crop up, then I think Newt has no chance. He is too far behind and would be the odd man out even in a brokered convention because he will not come in even second with Santorum winning Ohio, Tennessee and PA.
But if Romney or Santorum have to drop out for some unexpected reason, maybe he does have a chance?
Also, candidates may just stay in to make their policies relevant during the convention.
Or just to position themselves as VP.
Or maybe they just have delusions of grandeur?
To play spoiler?
This post is directed more about Newt then Ron Paul. But can easily be applied to Ralph Nader, Ross Perot or George Wallace.