UPDATED WITH A FULL TIP TABLE: I can always figure out the tip in my head for myself, however the real reason and need for a calculator (for me) has always been when we have a group of people and everyone wants to be fair.  Once you enter your bill amount, the calculator will do a regular calculation (if you want) and present you with options for 10%, 15%, 18%, and 20%, all rounded UP to the nearest dollar.  (I did this because I believe that rounding down is rather rude.)

However, if you want to split the bill, you can run the shortcut, enter your total bill, enter the number of people who will be splitting the bill, and it will present you with the same percentage options, slightly rounded to the nearest few cents (as you can see from the final bill amounts of different cents on the dollar).  This will allow you quick access to a full tip amount, split as fairly as possible.