All the hardware and software that's employed by an organization has potential security vulnerabilities that attackers can use to get into the network. These sort of attempts can have disastorous results for the victim organization. Information that can be crucial for individuals or groups can be copied, encrypted out of reach, or even worse; erased. Systems can be brought down to a grinding halt, data may be rendered unusable, and other forms of consequences can arise to demolish workflow.
Servers, lap and desktops, backup devices, smart tablets, phones, network equipment, printers et cetera need to go through some trials before they get plugged into your infrastructure. Hardened tests need to be applied, and overlooked details like changing default credentials and settings.
Also, it is crucial for new personnel to go through orientation and device security before they take a role in the workflow.