This can be misleading.
Earlier payments are not automatically applied to principal in many jurisdictions. They are instead immediately applied to payments. In other words, over the life of a mortgage, many payments are scheduled, and they can even vary if the mortgage is not fixed-rate. If early payments are sent without specification in some jurisdictions, principal & interest may not even be prepaid instead payments will be accrued, so the principal is not reduced, thus interest is still being accrued off of the unreduced principal, and the investors or processor receives an interest-free short term loan in the form of a "payments accrued" account. This serves to raise the interest rate for the borrower and lower it for the lender or processor. As payments are due, and no new payments are made, the payments accrued account funds the payment due.
Early payments must be specified to "apply to principal". They could even be applied to interest, but this would be a waste since it effectively raises the interest rate.
Paying the principal instead of a full payment or even interest alone will reduce the amount owed, substituting cash for equity assuming the value of the home doesn't fall, while keeping the interest rate constant. Paying any payment or interest early only serves to increase the investor's interest rate since that money is received faster and can be reinvested elsewhere.