It's pretty difficult to see a drone at distance (at 400 ft. it's just a speck after all), but it's pretty important to be able to see the air between you and the drone, and around the drone. That way you can be aware of any obstacles around it and can be fairly sure that you'll retain RC connection to control it. If you fly the drone over a ridge, or behind a building then a) you can have situational issues with what's behind or beside it (leading to a plummet & crash!!), or b) you could get a signal loss that leads to your drone doing something it shouldn't (leading to a plummet & crash!). In theory, we can pan the drone and rotate the camera gimbal to get a fairly good 360 degree view of what's around the drone, but we can't look at all those directions at once if the drone's in a piece of sky we can't see.