Sure, you can look at the source on your own machine.
There are times though where it's just more convenient to browse the source directly on Github. For example: if you're looking for a little snippet of code in some old/large repository you don't already have a local clone of -- it may take a while to clone it (slow connection, huge repository, etc.). Showing the file sizes in the "folder view" on Github would be helpful to guess which file you actually need to look in.
