4. Things you want to not run at runtime. If you can demand of the compiler that foo is run only at compile time, you can eliminate some performance outliers, with good protection against accidental later regressions. (I suppose this could apply to security too — anything run at compile time is definitely not vulnerable to crafty user input from a malicious user.)

5. My compiler is a rackmount somewhere; even if constexpr evaluation is somewhat slower than runtime code in a particular case, I may prefer to use the rackmount's plentiful power rather than the slim, light battery on the device where the final code will run.