What is the actual use of “signed” keyword?

There is only one instance where you might want to use the signed keyword. signed char is always a different type from “plain” char, which may be a signed or an unsigned type depending on the implementation.

C++14 3.9.1/1 says:

It is implementation-defined whether a char object can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char, signed char, and unsigned char are three distinct types […]

In other contexts signed is redundant.

Prior to C++14, (and in C), there was a second instance: bit-fields. It was implementation-defined whether, for example, int x:2; (in the declaration of a class) is the same as unsigned int x:2; or the same as signed int x:2.

C++11 9.6/3 said:

It is implementation-defined whether a plain (neither explicitly signed nor unsigned) char, short, int, long, or long long bit-field is signed or unsigned.

However, since C++14 this has been changed so that int x:2; always means signed int. Link to discussion