
Taking the back puts you at the top of the BJJ position hierarchy. Your chest is glued to their back, both feet are hooked inside their thighs, and your arms go one under the armpit and one over the shoulder to form the seatbelt. They can't see you. From here you open up the full menu of neck attacks (rear naked choke, bow and arrow, cross collar choke, ezekiel) and controls that draw out mistakes on their own (body triangle, choke threats that force them to defend the neck at the cost of an arm).
Back control teaches the cleanest rule in the system: control before attack, attack from control. Rush it and you lose the position. Pressure, patience and control get you there, not speed.