There are several advantages to buying Office 365, as opposed to any other version of Microsoft Office. While it might seem like a drawback to have to pay an annual subscription fee for Office 365, that does entitle you to some significant perks. For instance, you get access to OneDrive cloud storage service, which can provide you with a full terabyte of storage for all your Microsoft documents or other files, and you also get access to Office applications for mobile devices. Your subscription version can entitle you to install Office 365 on as many as 5 different devices, which could be very handy in an office environment where users have a traditional desktop and laptop or tablet.
When you buy any other version of Microsoft Office, it's true that you only have to pay once before you own it, but when a new version of Office comes out, you'll be obliged to buy that as well, or fall behind by some number of versions. You can also only install Office on a single device at a time, so if you need it to be on any other machine, you'd have to uninstall it from the original computer and reinstall it to another one - or, you'd have to buy a second license key.
Scalability is another big appeal with Office 365 - it can easily accommodate any kind of expansion your company might go through, without you having to make any changes regarding management of your documents. Cloud storage can easily handle a sudden increase in the volume of documents, allowing additional access by more and different types of devices with ease.