I'm in a postcard exchange network, Postcrossing, and so I'm sending postcards out to all nations of the world. Currently my sending limit is up to nine cards out there at once--everyone starts with a limit of five cards and you prove your devotion over time. Sometimes I have the appropriate postage and sometimes I've got a random assortment of stamps.
And sometimes I want to send a nice postcard that happens to be square. Square letters and postcards incur an additional 13¢ because they have to be hand-cancelled. Postal processing machines can't right the card/envelope into an appropriate position if it's not rectangular, so the post office has to tack on a little fee for the manual processing, yet the post office does not sell 13¢ stamps.
Enter the postage calculator. Type in all the denominations of postage stamps you have, type in your required postage, and it easily figures out how many of which stamps you need to meet the postage requirements. Sometimes you'll go over the amount, sure, but it's usually by only a few cents and it's still easier than driving to the nearest post office and buying the exact amount of stamps.
Or maybe it's not. If it's not, you should definitely patronize your local post office. If it's an inconvenience (like for me, between the holiday crowds and all the streets being coated in glare ice), go ahead and use up some of the extra stamps you have lying around.
Q: Is it easier to type in all the denominations of postage you have or to do the simple math and figure it out for yourself?
A: Go away.