What are the flags/banners on the hills for?
Solution 1:
The banners on the mountain tops are linked to a challenge that comes with the battle pass. You go to them and collect them, after which you will gain 5 battle stars as a result. This is only important if you own the battle pass.
This was introduced in week 6. I have added a link that shows you where they are and what they "Do" so you can get a better picture.