Sometimes for me, the chord progression or riff puts constraints on what will work lyrically so it forces me to come up with more original lyrics and melody.
On the Snakes & Arrows MVI by Rush, Geddy and Neil talk about the way they write. The musical structure is most often done first and if Neil has something that will fit, he gives a draft to Geddy who looks if he can actually sing the words the way they are written (considering syllables, breathing, etc..) and also has to find an emotional connection so he can deliver a good performance.
Most often Geddy does not end up singing an original draft, but he will re-write it several times, or take pieces from other things Neil has.
If no lyrics work out, it becomes one of their famous instrumentals.
Just though it was interesting...