Some applications need very specific drivers and protocols that aren't compatible with normal chips. Or you have to connect to a device via WiFi but still need internet. Also long range WiFi antennas are amazing.

Maybe the preferred Linux distro doesn't work with them. I had to use another distro for a while because Debian didn't immediately support the card, but there are apparently cases where the internal card just permanently wouldn't work (like in fully free software distros). I would rather replace the card inside the laptop than use a dongle, but idk if this can always be the answer.