Blocks are the easiest and most cost effective form of feed for rats. Rats scatter, hide and urinate on their food. Which can grow quite pricey. Seed mixes are not good, rats pick through them and eat what they like - not what they need.
Pet store brand foods are sold in small portions for a large price. This is not good for a breeder of 150+ rats that need to keep costs down. Lab block feeds are often better quality than pet food too.
EDIT: Most breeders use a rack system. The blocks are nibbled through the galvanized metal grids but the rats cannot pull them till they are small enough. This method cuts down significantly on rat block expenses.