It can be dangerous for a person to self-test during or shortly after a drinking episode to determine if they are legal [alcohol-wise] to drive. This is because their alcohol level may still be rising at the time of the test. However, a breath test several hours later, such as the morning after a party, will help the individual to avoid driving when they may still be over the limit.
Managers of licensed establishments may consider having breath test equipment available for use by their patrons to give them an objective measurement when deterring them from driving.
Hotels may also offer the use of such equipment to guests checking out and planning to drive early in the morning, where they may have over-indulged the day or night before.
This self-testing facility may be offered by the establishment as a free-of-charge courtesy, or as a chargeable service to cover the cost of the equipment.