Capemgr was created out of a need for boards like the BeagleBone to be able to detect installed expansion boards at boot time and allocate appropriate resources (load drivers, allocate GPIO, reserve bus addresses, etc) on kernels which use Device Tree. Traditionally, a Device Tree was a static file (or set of files) which described a machine (often an Embedded SoC and the board it's installed on). This of course is not able to support dynamically changing hardware, such as the BeagleBone and its add-on capes.
Further, it is one of the goals of the BeagleBone to allow 3rd parties to create their own capes and to allow users to install and use these capes without re-compiling the kernel.
A key feature of Capemgr is the ability to load Device Tree fragments as overlays to the existing Device Tree at runtime. On the BeagleBone, each cape has a Device Tree fragment which describes it.
https://elinux.org/Capemgr
