In a nutshell, yes, soffit vents prevent moisture from doing bad things to your home. Greebo is spot on, you want soffit vents for your air intake.
Without a continuous flow of outside air, the attic heat in the winter will melt ice and snow at the top of the roof, which will flow down and refreeze when it reaches the cooler bottom of the roof. This forms an ice dam, backing water up into your shingles and roof, resulting in some potentially expensive damage.
You also want to draw air in from the soffits which are on the underside of the roof and exhaust it at the top, which requires at least the same, if not more soffit vent area than you have ridge/gable vents. This prevents wind blown moisture from being driven up into a ridge vent and into your attic. If there is a positive flow of air out the ridge vent from the soffits, this minimizes the risk of the ridge vent becoming an intake vent.
And finally any moisture that does accumulate in your attic will be quickly vented away by the airflow from the soffit to the ridge vents. This includes any moisture from the outside reaching the dew point and from any moisture from your home that escapes into your attic from any cracks.
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.
Best Answer
A vented soffit allows air to flow in. The ridge vent allow the air to flow out. The purpose of this attic venting system is to keep the roof deck at approximately the same temperature as the outside air across its surface so that melting snow won't form ice dams. The venting system is not there to cool the attic volume itself.
If the baffles are not properly installed so that they guide the incoming air up to the underside of the roof deck, air may be blowing through the insulation, a process called "wind-washing". This will displace loose fill insulation like cellulose and will reduce the R value of batt insulation which relies on still air for its effectiveness.