Even professional basketball players seem fairly unsure of the origins of the slang term "dime" being used to describe an assist in that sport. National Basketball Association star Steve Nash has described it as originating from the days when pay phones not only existed but cost 10 cents, or a dime, to use, with "dropping a dime" being synonymous with helping someone out. In the case of the phone call, dropping a dime is helpful because it allows someone to make a call, and in the case of a basketball assist, dropping a dime is helpful because it allows a teammate to score.
Assists are important plays on the basketball court, and this play is often recorded for statistical purposes. Records of figures, such as the most assists in a single career, or the most career assists, are kept for posterity.
In non-sports contexts, the phrase "dropping a dime" can refer to activities that are not helpful, so those who are familiar with this term from basketball may want to be careful when using the phrase when referring to something other than a basketball assist. For example, in some cases, "dropping a dime" refers to the action of ratting out or informing on someone. If one mobster tells law enforcement authorities about the activities of another mobster, that could be described as dropping a dime.