There's seldom a reason to do a factory reset. Why are you considering it? As to what happens, the Roku continues with all installed channels and any login credentials you've entered. It remains registered to your Roku account.
If you are selling, giving away, or otherwise discarding the Roku, you should do a factory reset to clear out everything I mentioned above. Beyond that, the only reason to consider a reset would be if the device is acting very badly and all other troubleshooting attempts have failed.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
