Back when we only ASP, developers who have to write webpages for forms knew
that the most tedious part is writing code to validate user input. users
input must be validated so that malicious use of the page can not be
reached. User input must be validated so that the piece is wrong
information will not be included. User input must be validated so
information stored is the standard. Yes, some people had libraries of ASP
functions to validate common things such as post code (zip code for your
America), e-mail address, phone number, etc. to see ASP.NET developers
boredom always have to check user input. They decided that in order to simplify
we live with, including validation controls.

ASP.NET validation controls also provide two ways
validation: Server-side or client side. The nice thing about these Validation
control is that it will perform client-side validation when it detects
browsers can (unless the client-side validation was disabled). thereby reducing
roundtrips. And it will do server-side where necessary. this
client-side/server-side detection and validation is performed without any extra work by