In the United States, the vast majority of mortgages are backed by the government or government-sponsored entities (GSEs) through purchase by Fannie Mae, Freddie Mac, or Ginnie Mae (which purchases loans insured by the Federal Housing Administration (FHA) or guaranteed by the Department of Veterans Affairs (VA)).
The payments collected by the mortgage servicer are remitted to various parties; distributions typically include paying taxes and insurance from escrowed funds, remitting principal and interest payments to investors holding mortgage-backed securities (or other types of instruments backed by pools of mortgage loans), and remitting fees to mortgage guarantors, trustees, and other third parties providing services.
The fee rate can be anywhere from one to forty-four basis points depending on the size of the loan, whether it is secured by commercial or residential real estate, and the level of service required.
Those services can include (but aren't limited to) statements, impounds, collections, tax reporting, and other requirements.
The value of servicing assets or liabilities is highly interest-rate sensitive due to the relationship between interest rates and expected prepayments (i.e., loan refinancing).