One of the best features of Apple's iWork suite (Pages, Numbers and Keynote) is the ability to easily collaborate with friends and coworkers. Rather than simply sharing your documents for others to view or critique or go off on their own tangent with, collaborating allows all parties involved to edit the document, and everybody will see the edits. It is an incredibly useful feature whether you are working on a Keynote presentation with coworkers or simply sharing a spreadsheet among friends to keep track of expenses on a trip.
Besides simply keeping large amounts of numerical data well organized, the true utility of a spreadsheet lies in its ability to easily manipulate the data to derive sums, averages, maximums and so on. To do this, you need to use formulas in certain spreadsheet cells. For a simple example, you could have the prices of all the items you've sold on Ebay in column A and their shipping costs in column B.
Unless you have been living in a cave since the early 90s you know that cyber security is a huge issue and that you should protect your data as well as you possibly can. If you are sending private information in documents and spreadsheets over the internet via email, Dropbox, iCloud or any other service, it becomes vulnerable, regardless of how secure you think the medium is. And who knows how the recipient will handle the info?
Numbers is Apple's spreadsheet application that, along with Pages and Keynote, make up the iWork suite. Chances are that the vast majority of spreadsheets you encounter at work or elsewhere are going to be Microsoft Excel files. Opening an Excel file in Numbers will change it to a Numbers file, so if you make changes to it you will probably want to save it as an Excel file if you plan to pass it back to your Excel-using co-workers. This can be easily done on your iPhone, iPad and Mac, here's how:
iPhone and iPad