just wonderinghouse traps/masons traps/running traps arent legal in many areas with dedicated sanitary only sewer systems. they went the way of the drum traps and cesspools - obsolete. still think its legit question? lots of advances in santiary plumbing since 1920s!
why? they tend to get sucked dry from other drainage activites downstream in the sewer and tend to hold solids/clog. weather/pressure changes and the trap empties like a barometer. back in the days when entire plumbing was within five feet of the single stack vent.
fumes in a septic tank and even in a municpial sewer can be under pressure the fumes simply can bubble through the water in the trap and cause the same problem.
traps and vents are related!
The purpose of the trap is to form a water seal which prevents sewer gas from entering the building.
The purpose of the vent is to assure that the water in the trap is not pushed or sucked out by the movement of water through other parts of the drainage system.
vents also break vacuums so water flows down the pipes faster. remember the catsup bottle analogy? they let fresh air in and foul air out and equalize pressures.
a running trap or house trap alone didnt do the job. this is why all fixtures non mechanically draining fixtures have to be trapped and vented now. when all fixtures are trapped and vented and the main is also vented the running trap becomes obsolete except for the cleanouts.