Shifters play an important role in your drivetrain as it impacts how your gears behave. If you think about it, the derailleurs, stripped to their bare minimum, are a spring with a chain guide. It's the shifter job to command the derailleur and allow it to move exactly what you want it to move. Shifters are therefore, intricate pieces of engineering that give you feedback of what's happening with your gear selection and how precise that movement is.
For ultimate drivetrain performance, you should always pair your shifters with the same derailleurs, for optimal cable pull and resistance.