Not sure if this applies to biology, but I’m using more statistics for this.
Basically, when you change the y-axis to 300, you are able to exaggerate the patterns on the graph. From afar, or starting the y-axis at 0, a person cannot easily identify the increases and decreases of CO2. However, with a y-axis at 300, you see these changes much more easily, so you can make accurate inferences and conclusions regarding the graph.
Hope this helps