IPP-over-USB allows using the IPP protocol, normally designed for network printers, to be used with USB-connected printers as well. This way driverless IPP printing and scanning, and also other advantages of network printers, especially the administration web interface can also be made use of on USB-connected printers, scanners, and multi-function devices.
This way you can set up your printer or scanner as easy as a USB stick also if you want to have it connected to USB. With this Snap installed it gets automatically discovered, apear automatically as print queue in CUPS and is automatically available in SANE-based scanning software. No need to find an appropriate driver for your device and you get the printing and scanning quality as designed by the devices manufacturer.
This works with most modern printers, scanners, and multi-function devices. Devices which do AirPrint/AirScan, IPP Everywhere, Mopria, ... do the same also on USB and so they perfectly work under Linux and nearly any other operating system. Usually printers which have both network/Wi-Fi and USB connection and allow printing from phones are supported and do also driverless on USB. Perhaps even some USB-only devices work, simply try.
Also the web interface comes in handy for configuring your USB-connected device. Especially if you have a Wi-Fi device without front-panel screen to configure the Wi-Fi. Instead of needing a proprietary tool to configure the Wi-Fi while the device is connected via USB, you connect the device via USB, but use the device's own web interface to configure its Wi-Fi connection to your router.
The web interface you find on http://localhost:60000/ (for the first device, if you have more, you find them on http://localhost:60001/, ...).
If you are used to the configurability of a classically installed ipp-usb, you find the (editable) configuration file under /var/snap/ipp-usb/common/etc/ipp-usb.conf.
Note: Do not use any driver-specific utilities (like the HP Toolbox of HPLIP) and do not try to create CUPS queues with drivers when using your printer via IPP-over-USB. Please remove old print queues if they do not work. For printer maintenance use the printer's own web interface.
Technically, this is a simple Printer Application, a software emulation of an IPP device. It fulfills all standards of IPP and therefore easily integrates with CUPS and SANE (and if you configure ipp-usb to share your printer on your local network, you can print to your USB printer right from your phone).