Three examples of how evidence may be destroyed by onlookers at a crime scene are these:
1) Onlookers may walk through the crime scene that may destroy the footprints of the criminal or the victim.
2) Onlookers may enter the crime scene and touch some item which will result in destroying the fingerprints found there.
3) Onlookers may touch, take, move around or change an object from the crime scene contaminating / destroying any fibers or hair which may come handy during investigation.