The accepted solution is incorrect. There is no "auto" setting. It's either on or off. For most people, it's a personal preference. Some are more sensitive/annoyed by a mismatched frame rate, and it's also dependent on the capabilities of the display.
Most people complain about frame rate matching because it causes the screen to blank for several seconds (or longer) while the Roku and the display sync at the new rate. That was the biggest reason channels like Netflix were locked at 60Hz, regardless of this setting. As the video began playing in the background, the screen would blank. Then when the viewer moved to a different title, the screen blanked again while it switched back to 60Hz, then blanked again as the next video preview began. I agree it was extremely annoying. A better solution would be for Netflix to stop with the background video playback, and I understand that is finally an option.
Most movies are streamed at 24Hz, although some are at 23.976Hz (these fractional frame rates likely go back to analog TV broadcasting limitations). Same thing happens at 60Hz, with some content actually at 59.94Hz.
Most modern TV displays are capable of matching the incoming refresh rate to the screen's native rate. There are also a number of displays that can accept a native 24Hz signal and display it as is. Sometimes it's better to have the player (in this case the Roku) send the content at the media's native rate and let the display convert it or display it as is, depending on the capabilities of the display. My suggestion is to try it both ways and see if one has any better imaging than the other. One note of caution: media at 24Hz is likely going to be a bit dimmer than the same content at 60Hz. So if you're in a brightly lit room it might be better to leave display rate matching off.
