Standardising the contract creates enormous liquidity in an asset class made of millions of heterogeneous loans. TBA is one of the most traded fixed income markets in the world and is how mortgage originators hedge the pipeline of loans they have promised but not yet closed.
Because it is a forward market, the price difference between consecutive settlement months, the dollar roll, embeds the financing cost and the expected value of the cash flows given up. Specified pools with favourable prepayment characteristics trade at a payup over TBA.
Example: a lender locks $200 million of 30-year loans at 6.5%. It sells $200 million of the 6.0% coupon TBA for next-month settlement, so a rate backup that cuts loan values is offset by the gain on the short.
Related: mortgage-backed-security, prepayment-risk, liquidity, hedge, negative-convexity