spring balances are a reliable and portable construction, which allows the user to weigh lighter stuff right at the spot. It consists of a spring and a hull made of plastic or other reasonably sturdy material. You have to pay attention when buying a cheap spring balance: These scales have no overload protection and can break from hanging too much weight on the spring.
All in all they are fine scales for the pocket. Before the age of the digital LCD-scale and pocket scale, the spring balance was a good mate in trading and weighing activities. The principle of a spring dynamometer is based on the determination of the gravitational force in newton. It gets detected by the strain imposed by the weight of the spring. However, spring scales display the value in grams or kilograms and not newton.
The calibration of spring balances does depend on the gravity at the location. The actual weight gets falsified from the varying gravitational force on different places on earth. Some may know the spring balance from the physics class in school or other teaching programs. A spring scale is really good to demonstrate different effects in physics. You can demonstrate the gravity for example, when attaching the spring balance to a small cart that rolls down a slope.