Each tab has its own set of use. Home is for signing in your MS account and viewing recent presentations, spreadsheets and documents; New is for creating a new presentation, spreadsheet or document with corresponding templates; and Open is for opening an existing presentation, spreadsheet or document on your smartphone.
The update is now live across PowerPoint, Excel, and Word for iOS. To see the changes, make sure you’ve downloaded the latest version at the App Store.
In related news, Microsoft has recently begun work on a new Office app that has PowerPoint, Excel, and Word in one. Currently, it’s still in beta testing within the TestFlight program and no word has been released about its launch.