Your skin is the first line of defense against disease - burn victims don't have any (over the wound) so they are extremely vulnerable to infection. If the skin cells intended for a patient are not cultivated in a sterile environment, then they may become contaminated by harmful bacteria which would then be sprayed on the patient's raw unprotected flesh significantly reducing the chance of survival. The process of growing the cells would also grow the harmful bacteria, and anything else, magnifying the effect over normal exposure such as through the air.
There is also the case where the tissue is intended to be used as part of a sensitive experiment - the possibility that measurements performed may be thrown off by contaminants can invalidate the work. Or maybe the culture forms part of a criminal investigation - one side or the other, at court, may try to have the results thrown out and one way to do that is to show that the sample had been contaminated.
It's possible to scare yourself thinking about this stuff.