Imagine that you are borrowing a large sum of money at some time in the future. It's not clear which way interest rates are going to move - they might go up, they might go down, no one seems to know.
You could take a variable interest rate that will rise and fall as the market rate changes. Or you could take a fixed interest rate which is 1% more than the current variable rate but which won't change as the market rate changes. Or you could take a mix of both.