Why do you need secondary market?

Why do you need secondary market?
In short: to provide liquidity

For example, before the secondary market for mortgages a bank would hold the mortgage and gradually get their cash back over many years. With the secondary market they can sell the mortgage, getting back immediate cash for the value of those future cash flows.

If I hold stock in a non-traded company I may have to hold the stock for a long time, hoping to benefit from dividends or finding someone to buy it from me. With a secondary market I could create a "sell" order and accept the highest bid in the market, receiving cash for my stock quickly.