In terms of usability, monochrome line icons are not particularly a good idea. At a glance they tend to look the same. They’re often abstracted to the point where you need to be familiar with the “vernacular” of app iconography to interpret them.

They are mostly used for aesthetic reasons. It wasn’t that long ago that icons were expected to use color and shading. For usability’s sake, we should hope some version of that trend returns.