At the most basic level, they can convert one USB-C port into multiple USB-C or even USB-A ports, allowing you to connect a bunch of drives, USB memory sticks and peripherals at once.
This is where USB-C hubs can be so useful. Where you used to be able to rely on having ample connectivity to hook up a display, mouse, keyboard and some kind of storage, now you’re lucky to get more than a couple of USB-C ports, one of which may double up as the power-in socket. Today’s slimline laptops don’t go big on ports and sockets.