Many homeowners mistakenly believe all floor drains need a trap primer to block sewer odors, but this is not true. A trap primer is only required for drains in specific situations.
Standard floor drains are fitted with built-in water traps. The standing water inside forms a seal to stop foul smells and pests from coming up from sewers. Yet the water will evaporate over time, breaking the seal and causing odors. A trap primer works to automatically refill water and keep the seal intact.
Most household drains in bathrooms, kitchens and laundry rooms see daily use. Regular water use constantly replenishes the trap, so a primer is totally unnecessary and a waste of money.
By contrast, trap primers are a must for rarely used drains. Typical cases include those in basements, empty houses and unused storage rooms. They are also essential in commercial and industrial places like malls, factories and underground garages, where unstable pipeline pressure speeds up water evaporation.
In short, trap primers are optional extras. Install them only for idle or commercial drains based on actual use.