The importance of concatenation of JS files is that it reduces the number of requests ta browser has to make before loading the page. The more request a browser makes the longer it takes for the page to load.
What is concatenation of JS files?
Concatenation of javascript files means merging different JS files into one. For example, if you have comment.js, form.js, and register.js, you should combine the three and have them load from one file called combined.js.