What is the use of combo box in the excel...

It is used to restrict the values allowed in a cell. Instead of being able to type anything, clicking on the cell presents a list of items you can choose from.

Very good one

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

The use of a comobox or pull down in any interface is to allow the user to choose from a list of items, rather than type in a value. This is genrally used to standardize input.