Why do jeopardy players wager so much money in the last question when they leave...

[rarespecimen32]

...their answers blank? Even when they're unsure of the answer or just leave it blank "ie. What is...?". Why doesnt the winning person just wager nothing and win the game? assuming one is leading at 20,000, the others 18,000 and the first one wagered all her money and left the answer blank, whereas the other one didnt know the answer and didnt wager anything (which seems like the logical thing to do)!
someone explain this to me please

 

[Kitten]

They wager the money before they get to see the question.They are given the Topic of the question,they wager THEN they see the question.So based on the topic they decide how much to wager without seeing the actual question