I don't quite understand the question, to be honest.
You use the Key Change when the key of the music changes and Meter Change when the meter changes.
Those have always been some of the fundamental things in composing.
They are not gimmicks but normal tools for a composer, daily tools for one who does
other than rock/pop. Notation can get really fiddly if you don't know how to use those (I don't :o)
People use these features just like you use a screwdriver to attach a screw.