One of the big selling points of electric cars these days is they need less maintenance than an internal combustion-powered vehicle -- but low maintenance isn't the same as no maintenance, and that's why you might need a dipstick in your Tesla.
Let me explain. While there isn't a traditional oiling system in a Tesla like you'd find on an ICE-powered car, things still need lubrication, especially the gearing attached to those big honking electric motors that give Teslas their famous turn of speed. The thing about oil is that it tends to not want to stay put and even in a mostly closed system it can migrate elsewhere, thus reducing its level where it's supposed to be below what's desirable, which is why you (or in this case your Tesla service center) need a means of checking it.
Sure, you can check the oil level with electronic sensors, but that is an expensive and needlessly complicated way of dealing with a simple problem (We're looking at you, BMW!). To us, Tesla's decision to go with a simple piece of kit like a dipstick is actually pretty refreshing when you consider the way the company usually does things, "falcon-wing doors" for example, even if it's something that a shop is meant to use rather than the driver.
An interesting fact is that this dipstick (discovered by Bozi Tatarevic) is specifically listed in the Tesla master tooling list as being for the , which would lead us to believe that there may be no provision for checking the oil level in the drive unit on the Model S or .