Extractor fan advice

Hi there.

I am about to have an extractor fan put in and have been doing a bit of research as to which one to get.

Apparently, new builds tend to use extractors that are constantly on (be it at a very slow speed) but then ramp up whenever needed.

Is the a continuously-on version worth considering or should I stick with the old type?

Continuously on means continuously wasting electricity. Why would anyone want to do that?

Exactly - plus, would it not be pulling dry warm air out of the bathroom unnecessarily too?