With a mortgage, you're essentially leveraging around 1:5, which means you put down your 20%ish and get 100% of an asset. If the asset moves down 20%, your net equity is worth exactly 0. If the asset moves up 20%, you've made a profit of 100% (simplified, not considering the fees and other costs).

The trick to understand this is, if you put your down-payment money somewhere else, like a long-term index fund, what kinds of profits would you have expected? What about the risk, considering you're highly leveraged in a mortgage?

Thus the calculator to do the math for you.