When I'm composing, I'm not often using 'random' notes, looking for one that sounds good. I'm probably not thinking 'notes' at all, usually. I'm looking for motion, transition, joining melodies, arpeggios and sections. For this, amongst other 'tools' such as experience, feeling and luck, I would often call upon the Circle of Fifths to give me a connection. I may not use it, or I may listen to several directions offered by this; I may change tack completely, but it's useful to have this option. Indispensable..? Not at all, but it's an extra string to my bow, which I can use if I want.