With Windows 8 and 8.1 Microsoft had a OneDrive app as well as the desktop sync client having the much missed placeholder system. With Windows 10 a OneDrive sync client app is pre-installed without placeholders so you have to sync selected folders or all your OneDrive files on a PC, so there is no way to open or access individual files without pulling down the whole folder.
With the new OneDrive app you can view your entire OneDrive collection without having to sync to a device which is very handy on devices with limited amounts of storage. You can open a single file without having to sync the entire folder and it is touch friendly. You can’t use the app offline but it is certainly going to be useful on my small tablets.
More details on the Microsoft Office Blog and you can find the app in the Windows Store. On my release build of Window 10 the store said the app wouldn’t work but on my Redstone machines it installed fine.