Cost Savings vs. Internal Staff

Dedicated Staff
Financially this makes sense for larger companies that can handle the overhead of an additional employee. You are also entrusting all of your IT needs to just one person. If this individual were to leave, he or she takes critical knowledge with them, and your business could potentially have many more problems if proper documentation was not up to date, or kept at all. Then the cycle of hiring and training someone new begins.

Multi-Hat
This person is not able to focus on the job they were hired for. Not only that but you're expecting this one person to know every complexity of your network. . If this individual were to leave, he or she takes critical knowledge with them, and your business could potentially have many more problems if proper documentation was not up to date, or kept at all. Then the cycle of hiring and training someone new begins.