Why do crime scene investigators build a frame around the shoeprint before casting?

A crime scene is a location where a crime took place and is usually well protected to prevent tampering of the evidences which can be gathered to investigate and find the culprit.

Investigators build a frame around the shoeprint before casting to prevent any unfortunate destruction of the evidence while the crime scene investigator is inside doing his work.All the impressions are documented according to its position and surrounding areas.

It also helps in holding the impression better and longer before casting takes place.

it would affect precipitation by causing flooding in some areas of the world due to climate increasing, causing the glaciers in the northern and southern pole of our planet to melt.

i wish you would have provided choices to simplify it and allow me to give an exact answer.