As pointed out, they don’t use it. However, there are loose plan for KWin to migrate to wlroots one day, and in fact a hostile fork exists that is exactly that (KWinFT). So a compositor can make use of wlroots to implement Wayland functionality, sway for example does exactly that, unsurprisingly since they’re sister projects by the same author.
It should be noted that libwayland (mentioned in the patch notes) also exist, and wlroot actually depends on it, so I guess libwayland is like the lower level stuff while wlroots saves you some work to integrate libwayland into your compositor; the motto is “Pluggable, composable, unopinionated modules for building a Wayland compositor; or about 60,000 lines of code you were going to write anyway.”