Compare Down Payments
You want to compare purchase scenarios and know the difference in payments with different down payment amounts.

Compare Rates
You want to know the difference in payments and interest paid over the term with various rates.

Fixed vs Variable vs ARM
You want to understand the difference between fixed, variable, and adjustable mortgages. You also want to create a prime rate increase/decrease projection and see if switching to Variable can be beneficial now or if you should wait for it to drop later.

Switch now or later
You want to know the difference in cost and whether you should stay with the current lender for the remainder of the current term, break your mortgage today, switch to another lender, or wait three or six months. 

For example:
Current Lenders (2 years remaining)
Option 1: Switch to the new Lender now (low rates)
Option 2: Switch to a new lender in 3 months (lower rate)

5 year fixed vs 2 + 3 year fixed
You want to compare whether you should lock into a 5-year or a higher 2-year rate. You anticipate that rates will drop in 2 years and plan for a 3-year term. You also want to calculate the break-even rate to see if it is attainable.

Fixed vs VRM converted to Fixed after x months
You would like to know whether you should go fixed or variable, and how much you would save if you increased your payments after converting the variable into a lower fixed rate in x months. You will also try to match your second term payment with the option one's fixed rate for maximum savings.