BMI, ASCAP, PRS and the many other similar organisations around the World collects license fees for the public performance & broadcast of musical works: they are known as "Performing Rights Organisations" (PRO's for short). They deal with music played on TV & Radio, live performances in a club or stadium, ringtones on a mobile phone and even the radio playing in the background at a supermarket.
Fees are collected from broadcasters & public venues and paid to Publishers and Writers. Nearly every Publisher and serious Writer in the World will be a member of a PRO because otherwise they would have to deal with each company in the World that is likely to use their music - an almost impossible task. Therefore, a PRO effectively acts as an intermediary "clearinghouse" between those who wish to use music and those who own the rights to it.