Because crucially it rules out any failure with the aircraft. Yes we are certain it came down with a missile, but this will add further proof, that in this difficult case where blame is being passed around.
I'm sure there is also plenty of other information that can be determined by experts, such as as the exact position it was at and what the true radius is likely to be for debris. Plus whether the pilots had any idea at all, or whether it struck part of the plane before destroying the rest of it etc. Perhaps the missile also shows up on their radar? I don't know much about that, so its just an assumption.
