They buy the mortgage servicing rights. That is, they receive the payments and pass the money along to mortgage holders and, I would imagine, the municipalities collecting real estate taxes. They receive a fee for this. That's kind of an oversimplification but you get the idea. These rights were formerly held by the big banks but with changes in accounting rules are selling them off. If mortgage rates go up, as they are, it slows down the number of homeowners pre paying to refinance. This is good for a firm like HLSS which doesn't want to see their customers drop off the books by pre paying.