Hi. I'm not going to quote figures here because I'll have to start charging😜.
Firstly, liquidity. The money is tied up in the house, end realising this capital is a long, expensive, and disruptive process. So if you are mortgage free, but zero in reserve and the boiler packs up. You can't sell a bedroom to fund your boiler. You'd have to go into debt to fund the boiler. And that might be difficult, expensive or even impossible depending on the circumstances. Yes, eventually, your surplus income will help build this, but you will be initially quite vulnerable, and it will take some time to get back to the liquid position you had previously.
Secondly, return. If the return on savings/investments exceed the APR on the mortgage, you'll be better off.
Thirdly, savings discipline. This is a bet more woolly, but if your mortgage goes out each month as a non-negotiable, arguably, you'll repay this faster than you'd accumulate savings. Depends on how good you are at saving.
I know people have an instinct to repay debt, and that is good. But its not automatically the right choice, especially if it leaves you with little or no emergency cash.