What is the advantage of GCC's __builtin_expect in if else statements?

What is the advantage of GCC's __builtin_expect in if else statements?

c linux gcc built-in

c

Performance of built-in types : char vs short vs int vs. float vs. double

c++ c performance built-in

c

How to add a builtin function in a GCC plugin?

c gcc built-in gcc-plugins

c