My house has both septic venting that goes up through the roof and also a vent in the yard.
The roof vent is standard, with all the fixtures passing through traps and then draining down eventually through the basement as well as venting upwards.
The yard vent sticks up out of the grass about 16' from the house. It is just upstream of an underground trap which feeds directly into the septic tank. (This is not a municipal sewer.)
As part of another project in the area I have temporarily replaced the yard vent cap with a solid cap, so there is no airflow, and notice no difference on any plumbing in the house. Maybe there could be some other effect eventually?
What is the need for the yard vent? This came up because we are looking into relocating it, being in the way of a deck project. Is this vent a requirement (code or otherwise)? A "best practice"? Or ... ?
In reading around this subject I'm not finding yet anything reliable or pertinent for my system. There are discussions of venting the leach field or the septic tank itself, but my vent is neither of these. Being upstream of a trap I believe it provides at most pressure equalization as air won't flow through the trap by design.
Here's a photo of the vent when it was installed, just to clearly show the relationship to the house outlet (right), trap (center), and tank (left):
I've noticed that some homes have these vents, others do not, that's part of the motivation for the question, to understand the reasons for the differences.