Window tinting film question. Professionals or those with experience preferred.?

I purchased some window tinting film for my home. On the directions it says that the film should not be used on windows with chips or cracks. Does anyone out there know why this is the case?|||There are several factors that are at play. Certain window films can create theram stress on the glass. The existing crack can worsen. The second issue is aesthetic. The crack will look worse with the film on it.