Apart from screen mirroring and sms that everyone has mentioned the proprietary alternative undeniably has more features.
I don't know whether it is phone link or link to windows but one of them has a feature where your phone can be used as a webcam over WiFi. As someone who set up ip webcam on my phone and used obs virtual camera through most of the pandemic's online classes it's quite useful and I'd like to see features like that just work and are not relying on mjpeg and proprietary software on the phone end.
Their auto hotspot feature i haven't gotten to work and i think is useless but I'm thankful that i don't have to turn off kde connect when using samsung dex.