Why does Chrome need access to Bluetooth?
Modern Web APIs allow web sites to run code that communicates with your Bluetooth devices using the Web Bluetooth API - after you have given permission in each specific instance.
This particular prompt gives Chrome access to Bluetooth, which is necessary for Chrome to be able to offer that functionality to web sites.
You can find some technical examples of Bluetooth usage made possible by the Web Bluetooth API here:
Web Bluetooth Samples
In regards to actual use, it could be useful for controlling almost any kind of Bluetooth-enabled device. Imagine for example a new smart home device that you could setup by visiting a web site - instead of having to install an app or similar. Other examples could be RFID scanners, receipt printers, busy-lights, and so on.
Here's a GitHub repository with code examples for various fun demos:
Web Bluetooth Demos
For example, it includes controlling racing cars, a toy plane, a receipt printer and an LED pixel display.