For how detailed and insightful his analysis was -- which didn't elaborate or even touch on his not understanding the reason for adding the semicolon -- it sure appeared like he knew what it was for. Otherwise, the whole issue would/should have been about how he didn't understand that, not on how to keep adding the semicolon but do so in a faster way!

Then again, this comment from 3 months afterwards, indicates he may not think they are even necessary: https://github.com/rails/sprockets/issues/388#issuecomment-252417741

Anyway, just in case he really didn't know, the comment shortly below partly answers the question:

Since the common problem with concatenating JavaScript files is the lack of semicolons, automatically adding one (that, like Sam said, will then be removed by the minifier if it's unnecessary) seems on the surface to be a perfectly fine speed optimization.

This also alludes to the problem: https://github.com/rails/sprockets/issues/388#issuecomment-257312994

But the explicit answer/explanation to this question still remains unspoken: because if you don't add them between concatenated files -- as I discovered just to day -- you will run into this error:

   (intermediate value)(...) is not a function
       at something.source.js:1

, apparently because when it concatenated those 2 files together, it tried to evaluate it as:

   ({
     // other.js
   })()
   (function() {
     // something.js
   })();

It makes sense that a ; is needed.