WD40 was originally marketed as a water displacing chemical, and the name "WD40" stands for the 40th formulation tried for Water Displacement (which just happened to work much better than the previous 39 formulations tried). It was used by the US military to keep the nose cones on the missles stationed in Florida during the 1960's Cuban missle crisis from rusting. And, it still works well as a physical barrier between iron and the humidity in the surrounding air that would make that iron rust.

But, the reason why it works to protect metal from rusting is that WD40 leaves a residual oil film on the surface of the iron that prevents the humidity in the air from forming condensation on the surface of the iron and causing it to rust. Instead, that condensation forms on the surface of the oil film, and harmlessly runs off of that oil film, thereby protecting the underlying metal from corrosion.

My feeling is that a silicone lubricant will provide as good or better lubrication than WD40. However, because WD40 leaves behind a residual oil film over metal that's subject to rusting, it's a great product to use INDOORS to prevent metal from rusting. I used it on the iron piping of my hot water heating system whenever I would clean the rust off of any part of it, and the oil film remaining behind would act as a physical barrier between the exposed warm iron and the humidity in the air, and it's that physical barrier that prevented the exposed warm iron from rusting.

Outdoors, WD-40 is washed off by the rain, and therefore doesn't provide the lasting protection that an industrial coating (like an epoxy paint or nickel plating) would. So, nowadays, no one uses WD-40 to prevent metals from rusting since industrial coatings that work better and last almost forever are economically available. So, the makers of WD-40 market it as more of a lubricant than a water displacement chemical.

The rollers and tracks of a garage door would, in my view, be considered and "indoor" application since the door is only open for short periods. Most of the time, the door is closed and the rollers and tracks are not exposed to rain or sunlight. So, WD40 shouldn't wash off the rollers and tracks of a garage door as quickly as it would off of a bicycle chain, for example.