What is the thunderbolt port used for?

There are several articles about Thunderbolt, its evolution and changed specifications, goals, etc.

An extensive search can reveal much; if you have stamina and are doggedly good at tracking.

Here's one article that seems go generally in the main direction. While not fully conclusive:

• What Is Thunderbolt? - Here's Everything You Need to Know ...

https://www.digitaltrends.com/computing/what-is-thunderbolt/

What is Thunderbolt? Here's everything you need to know ... new USB connection

isn't compatible with Thunderbolt or Thunderbolt 2 without a pricey adapter ...

From about 2011 this technology has morphed but has not attained the lofty goals

in ultimate connectivity that it may, if adopted. The technology was intended to use

both electrical and optical; so far the electrical has won out.

• Thunderbolt - Apple

https://www.apple.com/thunderbolt/

Thunderbolt 3 offers a connection with state-of-the-art speed & versatility. Delivering

twice the bandwidth of Thunderbolt 2, it consolidates data transfer, video output, and

charging into a single compact connector. ~ And with the integration of USB-C, conven-

ience is added to the speed of Thunderbolt to create a truly universal port.

Thunderbolt is an interconnect technology developed by Intel in cooperation with Apple. Thunderbolt combines PCI Express and DisplayPort into a single connection, allowing for a combination of up to six peripherals, like storage devices and monitors, to be daisy-chained together.

You can also connect USB and FireWire peripherals via Thunderbolt, as long as you have the proper adapter. Those connections won’t run any faster just because they’re on a Thunderbolt connection.

You can connect additional Thunderbolt devices, or you can use it to connect to the computer with a Thunderbolt cable if the display's cable is too short or damaged.