What's the usecase of gcc's used attribute?

Attribute used is helpful in situation when you want to force compiler to emit symbol, when normally it may be omitted. As GCC's documentation says (emphasis mine):

This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.

For instance, if you have code as follows:

#include <iostream>

static int foo(int a, int b)
{
    return a + b;
}

int main()
{
   int result = 0;

   // some inline assembly that calls foo and updates result

   std::cout << result << std::endl;
}

you might notice, that no symbol foo is present with -O flag (optimization level -O1):

g++ -O -pedantic -Wall check.cpp -c
check.cpp:3: warning: ‘int foo(int, int)’ defined but not used
nm check.o | c++filt | grep foo

As a result you cannot reference foo within this (imaginary) inline assembly.

By adding:

__attribute__((__used__))

it turns into:

g++ -O -pedantic -Wall check.cpp -c
nm check.o | c++filt | grep foo
00000000 t foo(int, int)

thus now foo can be referenced within it.

You may also have spotted that gcc's warning is now gone, as you have tell you compiler that you are sure that foo is actually used "behind the scene".