Venting a drainage system is one of those conceptual things that first-time plumbers often miss. The idea of maintaining a barrier between the sewage system and the house is an essential element in a sanitary public plumbing system. Without a trap to seal off plumbing fixtures, nasty things such as explosive methane gas and vermin can find there way into the house. Without venting, traps can be emptied of their seal by siphonage. Newcomers to plumbing installation don't always appreciate this fact.
Every fixture requires a properly sized vent.
These problems don't only fall on the shoulders of owner-builders. A few years ago, I was inspecting the work of a general contractor. He was a grizzled old veteran of the contracting wars. An unlit shredded stub of cigar punctuated the corner of his mouth. His round solid head rested on thick shoulders.