What is the reason for adding fillers to grass seed? [View all]

I've been trying to fill in bare patches (lots of them!) in my lawn, and noticed that some grass seed is 50 percent filler. Why? Is there a reason other than the manufacturer being cheap and trying to provide less?

Pennington's seed says they have no fillers, and I read that some Scott's seed can be almost half filler. Scott's is one of the big names, so I wondered if there's a reason for adding filler.