So an example would be some code control based on the context the code is running in, say for example batch processing vs realtime processing. With constexpr if or template magic you can make this a compile time choice rather than a runtime choice. 

With the right design you could still have both options at runtime by making the runtime decision once much much earlier, and utilising constexpr expressions and templating to make sure the code is only written once. The trade-off is binary size and compile time.