issues at lesson 11

I follow all the steps, but when I run SET "NODE_ENV=production" & NPM start, there are some errors like :
Error: Cannot find module 'webpack' 
I find a lot of ways to solve these errors, finally i run npm install webpack --save-dev, Upgrade webpack from 1.12.13 to 1.15.0  then it works. 
I don't know why I can run normally after upgrading webpack. Can anyone tell me why?
