Maple syrup can grow mold pretty easily, so once it's been opened and exposed to the air, I keep it in the fridge.

It is harder (but not impossible) for mold to survive and grow in cold temperatures, so it'll grow quickly at room temperature in the cupboard, slowly in the fridge, and basically not at all in the freezer. A bottle for me lasts months but not years, so into the fridge it goes!

Now to be fair, that mold isn't toxic, and it's fairly easy to remove by just scraping out what you can, and letting it boil to kill it off and expose the remaining mold spores so you can pick them out. It's just I'd rather not waste my time doing that if I could have just put it in the fridge anyway.