It's all about the ether.
Break metal line shown and you'll smell it, and once that rabbit is out of the hat, you're done. You need the ether so it can heat-up and move the temperature needle.
This mechanical wonder is a great replacement should you ever snap your hose (which is easy to do when ever you try to remove the probe from the head or radiator)! It's a new reproduction of the exact combo gauge used on many British classics. The letter/type style on the gauge face is not an exact match for Bugeyes but it's very close, and if you don't have a gauge to rebuild, this is a great option!