The very main reason is to have a higher quality audio output. Instead of having a shitty jack a device like this will have a very good DAC & amp, sometimes even a bi-DAC and bi-amp, and also a lot more power to handle bigger headphones.

Globally if you just have a basic pair of headphones / earphones you're better off having a USB-C <=> Jack adapter or a simple bluetooth receiver, but for cases where you want to have a really higher quality source then these will do incredibly well.