Some people leave their garage doors partially open in the summer for ventilation.
For example if someone drives their vehicle and the engine is hot and then they park the vehicle in the garage when it's hot outside the heat can build up in the garage more quickly.
So by keeping the garage door open a few inches or so it allows ventilation into and out of the garage and helps keep the garage cooler and allows the heat to escape.
A better option however would be to crack open a few garage windows though if your garage has windows.
I used to wonder why I would see my neighbors garage doors open every summer and they were only open a few inches or so.
I asked one neighbor why he did keep his garage doors partially open when it was hot out and he said it was for ventilation.
You could also add a roof vent to the garage roof to allow heat to escape upwards but keeping the garage door open partially is a good way to ventilate the garage.
Also when it rains and could possibly flood inside the garage some people will keep their garage doors partially open then as well to allow water to flow out.
Or if water is in the vehicle truck bed etc.