Many players for whatever reason are too lazy to memorize songs, lyrics, or have to use the charts as a big crutch,.. that's the first step....but after you memorize or just playing it out of your head by ear then you can ad-lib and play it the way you like instead of the stiff way it's written,....you gotta bring some musical soul into your playing and doing it by ear is a very powerful way to do it, your stage performance and appearance will be better also when you eyes no longer have to be glued to that chart,.......but if your playing with other musicians or in a big band /orchestra, etc, the charts are needed to keep the arrangement and players in sync otherwise it's a free for all..some may call it Jazz and that's ok with a combo of 2,3,4,5 where everyone can play & express their own thing and make it sound great,..
but in bigger player scenarios the charts are a must.