A while back, I switched from using a calculator (the calculator program on Windows, as it happens) to a spreadsheet (Excel). I've been very happy with the results; so has a friend of mine to whom I recommended this. It's probably obvious to most of you, but we didn't do it for many years after we got computers with Excel, so I figure it isn't obvious to some.
The main advantage is accuracy, and security about accuracy: When you're adding a list of numbers using a normal calculator, you can't be sure after the fact that you typed everything right. With a spreadsheet, you can see all the source data right there on your screen, and can check that you entered the right formula just by clicking on the total cell. And if you realize you made a mistake, you can correct the one wrong cell without reentering all the data.
Of course, you can also more easily save the data for the future, copy it into a document or an e-mail, and so on. But even if you know you don't need that, the ability to immediately verify that you've entered all the data right is enough to justify switching to a spreadsheet. And while some features of Excel and other spreadsheets may be quite complex, learning enough just to add a column of numbers is a matter of seconds.
So, again, this might be obvious to some of you (and perhaps useless to a few who don't have a spreadsheet program); but if my and my friend's experience is any guide, it's the sort of obvious thing that many nonetheless overlook for a long time.