High altitude means low air density. Ignition is [harder to achieve with lower density][1], and the size of the combustor determines up to which altitude and throttle setting an engine will stay lit. Testing that it really works at those low densities before flying is a really good idea.


  [1]: http://aviation.stackexchange.com/questions/10002/why-is-the-combustor-volume-of-a-jet-engine-decided-by-altitude/10014#10014