The best way to cut metal siding for soffit vents accurately

From the installation instructions

Face nail with a 2" 6d or siding nail.

For good measure you could put a dab of matching color caulk behind the nail head before you nail it all the way down.

Soffit vents ensure a continuous flow of outside air on the underside of your roof.

In cold climates, this prevents two critical issues. First, from condensation on the underside of the roof from moist air in your home or from earlier in the day that has made its way into the attic. And second, from snow that falls on the roof from melting at the top, and refreezing further down creating an ice dam that causes melt water to backup into your roof and ultimately into your home.

In warm climates, the main benefit is to cool the attic space to reduce your heating costs. Some building products break down in extreme heat. Insulation in your ceiling is only slowing the transmission of heat, but when you have a 120°F attic, you'll have a lot more heat transferring through the insulation than if it is only 80°F.

That said, older homes, and those built in warmer climates are less likely to have soffit vents. Between the gable vents and ridge vents, you likely have enough ventilation for your climate (not that more wouldn't be better).

For more details on roof vents, see tester's fantastic blog post.