IME there actually aren't lot of things WD 40 is the best, or in many cases even a good answer for, particularly in the electronics product realm. I use it to loosen/clean up rusty or otherwise contaminated parts. It will act as a lubricant and rust preventative in certain simple mechanical type applications as long as high speeds or heavy load carrying of the parts is not why you picked this for a lubricant.
For me it is not the best at difficult penetrating oil jobs or at heavy duty lubrication. It will clean up tools, free up a door hinge or a lawn chair folding mechanism, remove grease or rust from something and is a jack of all trades kind of product that is best when your task is on the simple, lightweight end of the spectrum. IMO using WD 40 on/in electronic equipment is not a repair technique, it's an accident that needs to be looked at by a technician. YMMV!