It is required to be evaluated at compile time, and it's const.

An optimizing compiler might see through a non-constexpr declaration like 'double a0 = ...' or it might not. Constexpr is somewhat more explicit, especially with more complicated initializer expressions.

The compiler can substitute the value how it sees fit. It's like #define, but type-safe and scoped.

Maybe it's folded into expressions, propagated through constant expressions, or used it in contexts that require compile-time constants (template parameters, array sizes, static_assert, other constexpr expressions).

I mean, not in this case of pi/2, where it's more about announcing semantics, but in general those are the purposes and uses.