Question

I don’t know, Why do we use Server.UrlEncode() & Server.UrlDecode()?!
in QueryString we see anything in URL, so why do we want encode or decode them?

Answer 1

The URLEncode method applies URL encoding rules, including escape
characters, to a specified string.

URLEncode converts characters as follows:

Spaces ( ) are converted to plus signs (+).

Non-alphanumeric characters are escaped to their hexadecimal
representation.

Also, I think you are talking about HttpServerUtility.UrlDecode method which decodes the encoded string and returnes you back the original string.

URL-decodes a string and returns the decoded string

URL encoding ensures that all browsers will correctly transmit text in
URL strings. Characters such as a question mark (?), ampersand (&),
slash mark (/), and spaces might be truncated or corrupted by some
browsers. As a result, these characters must be encoded in tags or
in query strings where the strings can be re-sent by a browser in a
request string.

UrlDecode is a convenient way to access the HttpUtility.UrlDecode
method at run time from an ASP.NET application. Internally, UrlDecode
uses HttpUtility.UrlDecode to decode strings.

More info …

Server.URLEncode

HttpServerUtility.UrlDecode