What is the purpose of having a mac connected to an iPhone/iPad through bluetooth panel

The main feature is that your phone can create a Personal Hotspot via that Bluetooth connection. I.e. you can use your iPhone as an internet connection for you phone.

It is not that common to use Bluetooth for that purpose today as creating that Personal Hotspot over WiFi is both faster and more convenient given the user interface details that Apple put into that.

Bluetooth can be used for many things - transfering files, creating a hotspot, connecting a device (like wireless headphones) etc.

A very Apple specific way it is used is for Airdrop. AirDrop uses Bluetooth to create a direct wifi connection between devices. If you've used AirDrop to transfer file between your Mac and iPhone / iPad, entries for the device can appear in each others Bluetooth list.

(Generally, to save battery, it is recommended to not keep Bluetooth on always-on mode, and use it only when needed).