The problem is many developers simply allow Apple to use the generic reason for their use of Bluetooth to track you. Pretty sure there's nothing nefarious about most. But they need to be more specific about why in the pop up. Here's an example of one with Dunkin Donuts. It doesn't tell you much.
One of the possible reasons is advertising in digital signage.
Since I'm in that field of work, I'll explain.
In the example above, you have the Dunkin app installed and you've allowed bluetooth access. You walk into your local Dunkin store. The kiosk in the store is also using bluetooth to look for the app. When your app "sees" this, it receives info from the kiosk and now you'll get a Dunkin notification of a special offer / discount code/ notification your favorite coffee or doughnut is ready / etc.
Lots of developers are requesting BT access now for this very reason and it's pretty neat. A local restaurant here uses it and when I walk in I get an app notification of all the Imperial Stout beers on tap.