My printer (well, PrusaSlicer) always starts with a line around the object I am printing. See (1) in the image. --Why? What does that do?
There is also this line (2) at the front of the bed. --Why? What does that do?
In addition to what is mentioned by others, it starts the flow going to get a smooth pressure in the nozzle. While heating up the bed/nozzle, there can be an inconsistent melt in the nozzle. Sometimes a bit will ooze out and leave the nozzle empty. The purge will even all that stuff out and get fresh filament in there before it starts actually printing