My Apple BT keyboards+mice run out of battery very quickly and sometimes randomly disconnect/reconnect. A newer non-Apple BT keyboard seems ok on its own, but then there are issues on the other end. My fairly new Windows PC can't wake up from Bluetooth keyboards/mice, so I have to keep a USB one attached. And the BIOS menu requires a USB keyboard too. Dongle eliminates all that complexity.

There are also some BT limitations at play. Key rollover is capped, and mouse polling rate is lower. But that'd only matter to me if I were playing PC games. Ability to switch devices is a plus for BT.