When should literal classes be used in C++?

Can someone tell me when are literal classes needed in C++?
I am getting a little confused from constexpr constructors, constexpr members, and I can't see what the point is. I'd like to see some practical use of it.

Also I'd want to know if a set member function needs to be constexpr, i.e.:

constexpr void set_num(int a) { num = a; }