What are the pros for defining a function to be constexpr. For example the gcd function

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

can be written using constexpr

constexpr ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

Is there any improvement in time it takes to run or execute the program or any other benefits?