Hiring your own security personnel requires a lot of work. This process requires the company to advertise the position, train the staff, and acquire a heavy insurance policy. It is easier to simply pick up the phone and inquire with a private security company that would love to provide you services.
With contract security, the business owner would simply call the private security company and request a new security officer. Reassignment is simple and quick, firm has other officers that can be pulled in to provide coverage during the absence.
When a contract is signed, there is a commitment and a trust which forms between the two parties over time. When contracting with a security firm, you are not only hiring a security guard, but the expertise of the management team.