To add some extra information.
Escaping is a way to add extra information to a sequence of symbols without introducing extra symbols. In order to do this, at least one symbol gets to function as the 'escape' symbol. This means the original role is lost and needs to be added using an escape sequence.
Example:
We have a language with 3 symbols: a, b and c. In which the symbols have the following functions:
Language 1
a has the function A
b has the function B
c has the function C
We now need to introduce the functions D and E, but we are not able to add extra symbols.
So we can do this using an escape symbol:
Language 2
a, is now the escape symbol.
aa has the function A
ab had the function D
ac has the function E
b has the function B
c has the function C
The sequence aabcabac, if interpreted using language 1 reads AABCABAC. But interpreted using language 2 reads ABCDE.
The problem with this is that, most of the time, you are not able to determine the language used from the symbols. This meta information needs to be provided before the data is processed.
The botom line is that you have no safe way to determine if a string is escaped enough.