One use case from this year: I got a lab result that was technically in range but had clearly plummeted since last year. So my doctor and I knew that something was happening (if my ferritin had gone from the seventies last summer to the thirties in March, then the twenties in April), even though the numbers for all three results were technically in range. It was clear that something was changing quickly and that I needed to start supplementing iron.
Another use case: I got a new diagnosis and saw a new specialist. She wanted to know what had been ruled out, so she looked over the chart to see which tests I'd had already. She could also tell if there was a test that had been done years ago and might be due for a repetition.