Usually the interest on savings accounts is about 2% lower than mortgage rates. And youre taxed.

Using made up figures, of you had 10000 dollars on savings you might earn 400 bucks in a year. You’ll get faced a hundred and end up with 300. With an offset, you’ll pay 600 less in interest and pay no tax on it. So effectively double the return.

So you have lost 300, in my example but never notice.