After every use. It's to keep rain and dust and other detritus off the grill so it's ready to go when you are. If you only put it on at the end of the season you're likely to have a rusty grill covered in dirt way quicker

Every use, unless you live in a desert.

I cover after every use, after cleaning it

You put it somewhere with a roof when your done. Those covers trap moisture.

Depends on the climate, time of year, and where the grill is located.

If it is humid but the grill is stored under an overhang then leaving the cover off might be better for letting it stay as dry as possible unless you get a cover that has ventilation. If it is out in the sun/elements then cover on between uses is going to be better.

UV light makes exposed plastic brittle. Mine would also get that gross pine sap coating if I didn't cover it. Humidity isn't a concern in the northern prairies, so even a decade old it has no rust despite being covered after...almost every use.

The words BBQ and Grill are used interchangeably where I live but either way this would remain true.

Every use.

this is highly location and circumstance dependent. Like we just had storms with possible hail yesterday. If you had started using it you might want to throw on the cover at least depending. If its completely out in the open with no cover at all then you should throw the cover on I would say. If its in a place with a kinda roof above it then you can be lazy and just throw it on if the weather looks bad or whatver.

I actually never completely close it, unless I'm cooking. Once done cooking, I leave it open to cool, clean it, and the leave the lid propped partially open.

It's still protected from precipitation, but if I close it all the way wasps inevitably build a nest in there. I don't like my hamburgers that spicy.

But yes, I do this after every use. How much effort is it to just cover it? Why wouldn't you?