Your Entire IT Knowledge Lives in One Person's Head
If your systems, passwords, and workarounds exist mostly as tribal knowledge held by one overworked employee (or a favor from your cousin who's "good with computers"), you don't have an IT department — you have a single point of failure with a vacation schedule.
When that person is out sick, quits, or simply can't answer the phone at 2am during an outage, the business stalls until they're reachable again.