Scouters are mainly used by the villains in Dragonball Z/GT but on occasion some heroes and friends will use them. The only time i have seen this is during the Saiyan Saga when Bulma uses the Scouter.
Scouters are used to tell what someones power level is but later on this is quite useless as almost everyone learns how to hide their power level. Most people can sense it but they do not know how much the person is hiding. Like Freeza can hide an emense portion of his power level. Goku is an even better example.
Scouters are also used for long distance communicators or short range. That is how Freeza and Vegeta learn about the Dragonballs, and how he knows to go to Namek