Having Cortana know your position can help her determine items which are most relevant to you, such as the weather, local news, departure, traffic and appointment-related notifications, and so on.
Position isn't a real battery drainer in this case since Cortana only needs a few moments to determine your location. In most cases, this location has already been determined automatically by the phone in the background so Cortana only needs to "read" this saved location (which takes a fraction of a second).
This is different from the case of using navigation apps, it is not the determination of location that uses up data, but rather the fact that the screen is left on while navigating and (most significantly) data downloads (in the case of Waze a live traffic updates). The GPS antenna does consume power, but not as much as those other two (the screen and the 3G/4G antenna)... and only if the GPS antenna is left on continuously (which is app-specific - the GPS antenna is only active if the location icon is visible in the upper left corner of the screen, which while using Cortana ins't really that long).