On some missions once you have done everything it will give you a secondary objective which is to land
Only on the ones where your near enough to a friendly landing strip though, such as the battle of britain campaign. Others are no so fortunate.
Theirs also a single player mission "inspect airfields" which involves landing at 3 different airfields (no enemies flying about) to complete the mission.
If all you wish to do is land then set up a training missions and have the arado bomber as your enemy. Dont arm it with anything and you have yourself a free flight as the arado has no guns what so ever.
Hope this helps a little.