Why are mpfr, mpc and gmp needed for gcc?

[René Nyffenegger <rene.nyffenegger@xxxxxxxxxxx>]

I hope this is the right place to ask this question. I only
recently tried to compile gcc and it is obviously dependant
on mpfr, mpc and gmp. These seem to be high precision
mathematical libraries. Now, I am wondering why a compiler
not only needs one of these but even three of them.