Further training might be required, but if there's a shortage of trained HVAC people, an employer might prefer to hire someone that already knows something about the field, and who happens to know a bunch of other stuff as well (because that indicates they're good at learning new stuff).

For the prospective employee, there's some advantage in not being deeply locked-in to one field in particular. Depending on economic circumstances, you might need to switch trades half-way through your career. The idea is that having some basic experience in a number of different skillsets will make a switch easier if you need to do that.

That's the theory anyway. I don't know whether it's really practical though.