Calculate allows you to make quick calculations without getting in the way of your current windows and workflow.
Sitting in your menubar it's always ready to use, it becomes available in an instant and disappears just as quickly allowing you to continue with what you were doing.
Calculate doesn't have memory buttons like a standard calculator which only holds one answer. Instead, Calculate remembers all your calculations including the actual sums themselves and displays all the information to the right so you can see everything at a glance.
Have you ever been in the position where you need to do a number of calculations to achieve the desired result? The main problem with this is you normally end up having to remember what the previous answers were whilst doing the next calculation but because Calculate keeps track of all your sums and displays the information, you no longer have to worry about remembering what the previous answers were, they are right there for you to see.