Since Office is Microsoft software it makes sense Microsoft would tie it to their own services. There are numerous ways to get OneDrive space, including an Office 365 account.

You can work on Office for iPad files locally on your device, with SharePoint links, or from a OneDrive account. If you're a student using the University version of Office online, you can get a four year subscription for around $80 - not a bad deal at all. The personal plan, when launched later in the spring, will cost a little less than $8 a month. There are several price points depending on where your needs fall but most subscriptions come in right around $100 a year. Your iPad shouldn't even count towards your 5 allowed devices either. If you've already got an Office 365 subscription, you're good to go and need only log in. The free versions do allow you to open and view documents at no charge. You'll need an Office 365 subscription in order to actually edit documents, spreadsheets, and presentations on the iPad. Office for iPad: Office 365 subscription pricing

Using microsoft office on ipad pro