Tome-In is to be stamped on the end of a document to clarify where a document ends when there is a blank space on it. If you write “This page is intentionally left blank.”, the same effect as Tome-In can be achieved.
When either one of contractee and contractor stamps and signs on a contract and send it to the other one to have him sign on it, there is a risk the other one may unjustly add extra sentence on it later. Therefore, you’d rather stamp Tome-In just in case to prevent this situation.