Reading on the AGP spec, the little bits I've found on sites like https://old.pinouts.ru/Slots/agp_pinout.shtml, say:
The Accelerated Graphics Port (also called Advanced Graphics Port) is a high-speed point-to-point channel for attaching a single device (generally a graphics card) to a computers motherboard, primarily to assist in the acceleration of 3D computer graphics. Many classify AGP as a type of computer bus, but this is something of a misnomer since buses generally allow multiple devices to be connected, while AGP does not. AGP originated from Intel, and it was first built into a chipset for the Pentium II microprocessor. AGP cards generally slightly exceed PCI cards in length and can be recognized by a typical hook at the inner end of the connector, which does not exist on PCI cards. Nowdays AGP is almost replaced by PCI-Express.
This tells me that the connector is, in fact, more generic and just a high-speed peripheral connect to the CPU. This has obvious advantages for graphics processing which needed the higher bandwidth of the time, but I can think of several other devices which would enjoy that connection speed (capture cards, disk controllers, cache devices, etc).
Were there reasons this wouldn't be done, and are there examples of hardware utilizing the AGP connector in this manner?