When I write music, I often scratch out the rhythms long before I write notes. That’s especially true with vocal music – I’ll layout the meter on a notecard and draw in rhythms that reflect my phrasing of the text. Then and only then will I experiment melodically within my rhythmic framework.
I use it when writing instrumental or vocal soli, as well. I’ll play the chords, or just audiate them, and be an amateur vocal percussionist, playing around until I find a rhythmic phrasing I like.
I do this always with Alice Parker’s guidance of not writing it down until it comes out of my mouth three times the same.
As I write my melody, I’m never afraid of revising my rhythmic writing; but having it as a framework to lay my melody in is a useful tool. It literally gives me scaffolding for my writing, often saving me from the block that comes from trying to do too much at once.
I also like this because it mirrors, to me, the development of music, both societally and as individuals – we respond to rhythm before we take in melody. It’s fitting to write them in the same order.