Here's why I think themed writing competitions or anthologies make good writing prompts.
They give us some direction and a deadline. Direction is good when we are stuck, deadlines are great even if we write regularly. We have our topic, we know how long it needs to be and we know when it's due. All we have to do is write our entry.
They might lead to publication and payment. Yes, we write because we love writing but let's be honest, we want to see our words in print. We want to be paid for our words. We are writers, we want an audience.