Sewer gas entering the home. With no vents the traps can siphon allowing the sewer gas from the septic tank or sewer line (what ever you have) to enter the home. That is the main reason but it can also cause the lines to drain slow. It's like holding your finger over a straw, the water in the straw can't escape.
repricussions of sewer gas entering the house could be death, or explosion as sewer gas is dangerous to breathe and Highly Flammablle in proper concentrations. The builder was an idiot, plain and simple.