There are some counties that require a pool alarm as well as alarms on your gates, windows and doors to be sure that someone doesn't sneak in for a swim. Most local codes require that you have a fence around your pool or in the case of above ground pools, a ladder that is removable or locking but may not require that you have a pool alarm.
An automatic pool cover will take the place of a pool alarm in most areas. Hot tub spas usually have a locking cover.
Most of us fall into the 'no alarm necessary' category but there are benefits to having a pool alarm even if you don't have toddlers in your household.
Since a pool alarm does not replace supervision, it just makes sense to be outside with children when you have a pool. Even then, a turned back could be just enough for a child to fall into the pool. Backyards tend to be noisy with lawnmowers, blowers, air conditioning units and radios. The combination can be devastating.
Even pool owners who do not have children living with them could benefit from having a pool alarm. The alarm is set off whenever the water moves, so it could protect a pet or let you know when there is any type of intruder.