Formatting the GoPro's SD card frees up space. But it also gets the card ready for use and keeps the filesystem healthy and minimizes the risk of errors. Here are the various ways to do it on different GoPro models.
While there aren't a lot of options for external batteries for the GoPro HERO6 Black and HERO5 Black, there are some. Here are the best options I've come across for powering your GoPro for more than a couple of hours at a time.