In a nutshell yes soffit vents prevent moisture from doing bad things to your home.
Soffit vents and gable vents.
While some of it exits through the gable vents and some exits through the ridge vent these outlets are not in competition with each other.
The cooler outside will be allowed to be sucked through your soffit vents and expel the hotter humid air through your gable vents.
Pros and cons of using gable vents.
As built the attic of our 22 year old house was ventilated with gable vents and soffit.
Greebo is spot on you want soffit vents for your air intake.
For an attic without soffit vents you may have the option to use gable vents instead.
Sealing and leaving them may look better from outside to avoid creating a blank gable.
If in this case you choose to add ridge vents you must either remove the gable vents or seal them up from inside the attic.
Normal vented air likes to enter the attic lower down near the bottom third of the attic and as it passes through it will exit higher up near the top third of the attic space.
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.
When gable end vents are combined with soffit vents this can keep a continuous air flow through your roof or attic.
Releasing all of the heat that rises and gets trapped in the attic can be achieved with one or a combination of the three following vent models in addition to the multipurpose gable.