If you write the words first, and then the tune, you can audition several chord patterns/tunes until you find the one that best fits the words.

If you write the tune first, you must limit yourself to the words that will fit within the template you've already defined by the tune.

I have a whole pocketful of tunes that "I need to write some words to go with....". The only ones I've actually written words to are some swing progression type things that are so "generic" that they needed words to give them more definition. I have never successfully put words to a truly original instrumental.