"But surely you have satellites which have excellent resolution, why not just use those"
Well from what I understand there is two problems with that, firstly certainly in the civilian world you don't have satellites with that ability, also you'd be repositioning a satellite to specific points all over the world, a costly and lengthy process for all of the world in that resolution, also you could have a satellite sat over Paris waiting for no cloud when London is clear, an aircraft is quicker and more effective at repositioning quickly to the clear sky.
wingisland, don't be too dismissive of commercial satellite imagery. The highest resolution available is a pixel size of 0.5m in black and white or 0.6m in colour. You don't reposition satellites, they orbit continuously and cover the whole globe in just over two weeks. With this repeat rate most areas will get cloud free coverage at some time.
0.5m sample can be viewed here:-
http://www.digitalglobe.com/download..._2007_dgwm.jpg
0.6m colour here:-
http://www.digitalglobe.com/download..._2005_dgwm.jpg
Not bad from orbit!