ASCAP, BMI and SESAC are Performance Rights Organizations (PROs) are your advocates in the music industry and an invaluable resource. These are the people who keep track of your music and make sure that you are compensated fairly for it. This includes airplay, live performance, film, television, commercial, theater, live events and streaming.
They lobby congress for fair pay for songwriters and performers. They offer insurance and other benefits.
They offer networking and educational opportunities, scholarships and awards.
They collect your performance and sales royalties, (even your own) leasing, and streaming $$$$.
There is no fee to join as a published composer, you might as well join all of them. (You must be invited to join SESAC, the European performing rights organization.)
Both ASCAP and BMI are about equal in their ability to service their artists. Each offer workshops, networking opportunities, scholarships and awards and, most importantly, advocacy.
SoundExchange is the organization tasked with collecting royalties from digital streaming.