Is it necessary to update Android apps?

Recently I got a new Android smartphone and I want to know if it's necessary to update Android apps frequently. The reason for asking this is 1) Does updating Android apps reduce battery usage? 2) Should I update my apps if I don't have much internal memory? Will it slow down my phone if there isn't enough internal memory? 3) Will there be any (real) performance improvement if we update apps? I know that certain updates provide new features, bug fixes (I hope it means errors), and changes in the background/layout. Without considering the added feature, bug fixes, and layout, is it necessary to update? Also, each update takes a good amount of internal memory in my phone as well as a lot of valuable mobile data.