Well they are just there to make sure that your package (downloaded or copied from somewhere) is not corrupted. After you first download or copy the package it is helpful to prevent you from installing a broken package.
A broken package may work or may not work (poor stability). I don't think we should care about security when the system is not even stable.
After the package has been installed, you often don't need to run the verification again, unless there has been a disk failure or a power outage (or unless you are really paranoid about it). In case of such incidents, the file system can be damaged, leading to broken packages.