Let us take an instance of where this can be used.
Many sites have user registration forms where the user is asked to choose a username and password.
Obviously this user login has to be unique and so the required validation has to be done to make sure that the user login is not already present in the database. This makes a server round-trip inevitable and hence will need the user to have one extra step in the registration process, where he needs to change his login name. Trying to bring this on the client side would mean that a JavaScript validation has to be done, but it is definitely not a viable solution, because by just viewing the source code, the user is able to see all existing login names in the database. We look at this method by which we validate whether a username already exists in the database. Here is where the server side programming techniques of ASP come into play.