Purpose of septic vents both in the yard and also through the roof?
These are different things.  The vent stack inside the house is there to prevent sewer gas from pushing back into the traps (as well as providing overall pressure relief).  The vent in the yard is there to handle gas generated inside the septic tank.  Whether that is required by local code, or as part of your specific design, or just because some people think it lengthens the tank and leaching field's life expectancy, I cannot say for sure. I have lived in towns where some people have added a tank vent, others have not, and there's no apparent rhyme or reason.