When Apple spent north of two hours dissecting all the awesome new stuff on the way for its computers and mobile devices, it split the iOS 11 segment into two parts -- one for iPhone and another for iPad. One of the most exciting new additions to the iPad in iOS 11 is of course drag-and-drop, but if you don't have one of Apple's tablets you'll still get to use it, because it's also coming to the iPhone.
On the iPad, multitasking is a big deal, and the ability to drag and drop files and objects helps bring Apple's slates more in line with desktop PCs in terms of utility. It's a huge addition, especially for those who rely on their iPads for work and productivity. On iPhone, drag and drop will work a wee bit differently.
As Verge reports, the drag and drop feature on iPhone is only functional within each app, individually. That makes a lot of sense, since the split-screen and multi-app features of the iPad don't work on the iPhone's much smaller display. That being said, the iPhone is also getting Apple's new Files app, which acts as a folder directory for storing whatever files you might need to manage and organize, and being able to drag and drop will be extremely useful in that regard.
Humorously, it appears that despite Apple's apparent intention that dragging and dropping be walled off to each app on its own, those who have been playing around with the beta have managed to make it work between apps:
Judging by how clunky this aspect of the functionality appears to be, and the finger contortions required to make it work, it would seem to be an unintended bug, and shouldn't be expected as a supported feature going forward.