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.
While Apple's iWork is a perfectly good productivity suite that includes Pages, Numbers and Keynote, Microsoft still controls the lion's share of the market with its own collection of office tools that are practically household names - Word, Excel and PowerPoint.
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?