It is used to decode the Escaped-encoding URL.

Escaped-encoding, or sometimes referred to as percent-encoding, is the standard of representing characters within a URI that may need special syntax handling to be correctly interpreted. It uses a sequence of three characters to encode the character to be interpreted. This triplet sequence consists of the percentage character “%” followed by the two hexadecimal digits representing the octet code of the original character. For example, the US-ASCII character set represents a space with octet code 32, or hexadecimal 20. Thus its URL-encoded representation is %20.

Applications may automatically escape reserved and unreserved characters, or automatically un-escape an escape-encoded sequence within a URI, if there is potential for it to be incorrectly interpreted by the remote application. This conversion may be due to the position of the character or escape-encoded sequence within the URI. For example, "%7e" is sometimes used instead of "~" in an http URL path, but the two are equivalent for an http URL.

Because the percent "%" character always has the reserved purpose of being the escape indicator, it must be escaped as "%25" in order to be used as data within a URI. The RFC for URI encoding recommends that care should be taken not to escape or un-escape the same string more than once, since un-escaping an already un-escaped string might lead to misinterpreting a percent data character as another escaped character, or vice versa in the case of escaping an already escaped string.

Useful Character Escape Code:
SPACE %20
< %3C
> %3E
# %23
% %25
{ %7B
} %7D
| %7C
%5C
^ %5E
~ %7E
[ %5B
] %5D
` %60
; %3B
/ %2F
? %3F
: %3A
@ %40
= %3D
& %26
$ %24