Does the scale read 100 N?
Similarly, what will the scale read?
When a person stands on a scale, the reading (the number of pounds or newtons) on the scale is actually the Normal Force that the scale exerts back towards the person to support the person's weight. This will change the contact force (the Normal Force) between the person and the scale.
Subsequently, one may also ask, what does the spring scale read?
The scale reads the tension in the string. The tension in the string is 100 N. The spring scale does not measure the net force. The spring scale simply measures the tension, the magnitude of the force exerted by the string.
They are used when the accuracy afforded by other types of scales can be sacrificed for simplicity, cheapness, and robustness. A spring balance measures the weight of an object by opposing the force of gravity acting with the force of an extended spring.