When I first joined my local writing group (over twenty years ago now!) I thought writing to a prompt was a lazy, hit-or-miss approach to getting a story. I thought, if you're a 'real' writer, you should have so much to say about life, you'll have stories bursting out of you.
I've since come to change that view. It seems to me now that writing this way is like going fishing - with the prompt as your bait. You throw out your line and hope for a nibble. If you get a bite and it's something interesting, you 'reel it in' by writing more about it, going deeper, exploring what's there.
This method works not because writers lack ideas but because they have far too many. Our choices for subjects are truly infinite. It's like asking someone, 'What do you feel about every event, every situation and every person you've ever encountered, real or imagined?' That is literally what every author has to work with. Hard to not be overwhelmed - where do I begin?
A writing prompt gives me something to focus on. How do I feel about the colour green, or a specific scent from my childhood? It's taking a single drop of water from an endless ocean and examining under a microscope. There might be something there or there might not. And sometimes we writers don't even know what we think about a subject until we write about it.
Don't get me wrong, ideas still come to me out of the blue. Stories and characters waking me at night, wanting to be written. But every now and then it's good to go off and do a little fishing on the side.