Raminaa,
In theory, char can use less space if the amount of space needed is known. This isn't a good reason to use char though; just wanted to mention it.
In practice, it is useful to use char if you care exactly how long a field should be. For example, suppose I am storing a US social security number. It had better be exactly 9 characters. If not, it is meaningless. Here, char(9) has business meaning and the database helps enforce it. If someone accidentally adds an 8 character
String, the database insert will fail - warning them of a data integrity problem.