use of captcha ,,why it used in every site before submit ?

It prevents from DoS attack as well as it make sure that entries is done by human being

Hello Santosh,
Captcha is used to avoid robot entries into the database, or the hack entries.

http://security.stackexchange.com/questions/26094/why-do-we-need-captcha-in-what-case-we-should-use-it

Look into the above link for more information from the discussion.
Thanks

hi, to prevents various bot to autosubmit the form. Captcha should be guarantee that the form was submitted by a human.