Homebrew PCBs

I have buying those 50 piece bit sets from Harbor Frieght Tools - good 
deal, you get a "blind" mix of drills and mills.  All around a good 
value to get some tools to get started.

However, I am wondering why there are always so many milling type 
cutters in these boxes.  Is there a lot of milling (not isolation type 
but through hole) done on PCBs?  I have never really looked for it in 
on PCBs I have seen, so I have no idea.

Chris