Aircraft could fly low for various reasons, Common reasons for low flight are sightseeing, photography, crop dusting, staying below Bravo/Charlie airspaces, flybys, and "just cause I felt like it". What are the difficulties and dangers faced by such aircraft?
A lot depends on how low you are and what aircraft you're flying, but any low-level flight has some risks and difficulties. Here are some, I'm sure there are others:
- If the aircraft loses power you have much less time and distance available to execute a forced landing, and your choice of landing sites will be more limited
- You can't see as far ahead so it's more difficult to look out for obstacles and pilotage may also be more difficult in some circumstances
- You may accidentally break the law on minimum safe flying height because the terrain changes too quickly, you don't see an obstacle or town until it's too late etc.
- There's a greater risk of collision with terrain or obstacles like radio towers, especially if your aircraft is a fast one and you have less time to react
- The likelihood of both thermal and mechanical turbulence is higher when flying low, increasing risks of loss of control and collision with terrain and obstacles
- If you're scud-running in VMC there's a high risk of entering IMC accidentally, which is extremely dangerous (incidentally, despite the title of that question, even instrument-rated pilots have a poor safety record when they enter IMC unintentionally)
- Radio reception may be poor
- ATC radar may not be able to see you
- Your engine may not operate very efficiently
That's not to say that low-level flying is always a bad idea, but it I think it does require more preparation, alertness and even local knowledge from the pilot.
Some of this depends on what you are flying as well, but as a rotorcraft pilot, we tend to fly at a lower altitude than planes. The number one killer of helicopter pilots is wire strikes. Wires, although they may seem obvious when you're on the ground, are next to impossible to see from the air. They're incredibly difficult to see both because the background is now the ground instead of the sky, and because you are likely to be quite a distance away (and if you aren't, its probably too late to do anything about it. You're either going to hit it, or not). You're also going to get quite a bit of mechanical turbulence, meaning turbulence that is caused by the wind blowing against things on the ground. Buildings, for example, will have air blowing upwards on one side of the building, and air blowing downwards on the opposite side. This air turbulence can extend quite a distance up from the object itself. This will be true for buildings, hills, mountains, etc. In the case of a plane, you're also going to get reduced performance (in most cases) because the air is denser, and you'll have higher drag. Helicopters have the opposite effect, where they become more efficient with higher air density, so they'll fly better near the ground and at low altitudes.