They create a map of your place and then clean it systematically instead of randomly like the cheaper Roombas.
This is obviously much faster, but also less thorough (lower number of passes over any given spot) and less robust (navigation easily gets confused by sunlight, dirty sensors, room changes, etc.).
If the device is smart enough, thoroughness can be a choice so the user can opt for multiple systematic passes instead of being required to wait through multiple random uneven passes.
> less robust (navigation easily gets confused by sunlight, dirty sensors, room changes, etc.)
This implies that robot vacuums that are sufficiently intelligent don't exist yet.