Why we use HTTP POST instead of HTTP GET

1. The total amount of characters in a GET is really limited (depending on the server)  .If the user types ,say. along passage into a "search" into a box,the Get might not work. 
2. The data you send with the GET is appended to the URL up in the browser bar,so whatever you send is exposed.Better not put a password or some other sensitive data as part of a Get.
3. Because of number two above ,the user can not bookmark a form submission if you use POST instead  of Get.Depending  on your app,you may or may not want users to be able to bookmark the resulting request from a form submission.