You need the battery pack only if you want more than the 2 hours of battery life the device itself gets.

ADDED. Since I am getting downvoted, here is a cite:

https://www.laptopmag.com/news/apple-vision-pro-is-here-and-...

>Up to 2 hours of battery life without the battery pack. (Yes, there's a battery back that can be attached to Vision Pro.)

Also: put yourself in the design team's shoes: why wouldn't you put a small battery in it? A small battery doesn't weigh that much; a small battery isn't much of a safety hazard; compared to all the other engineering effort put into the product, the engineering to put in a battery is a drop in the bucket (particularly since the organization has so much experience putting batteries in products).