Hard-coding a relative file path seems like an obvious example. For example a config file with settings. The file will always have the same name so heap allocations and mutability are not needed.
Then again, I am primarily a Python and C programmer, so there is likely some underlying reasoning I'm missing.
in C++ cannot change after construction, but is not compile time constant. This means that it has to run some code and allocate some memory at runtime.
In contrast constexpr is fully compile time and ends up in the read only portion of the binary. No code code execution or allocations necessary.