A poem with a plot that reads like a story. They concentrate on the rhyme but still tell a story.
You have the fun factor of rhyming and the value of a plot.
They fit perfectly into a story session. Do they work as bedtime stories? Sure - why not?
The simple plots keep your child engaged while still having the fun that a rhyme provides. And if ultimately it promotes sleep, then I'd say it's done it's job.
I hope you and your kids enjoy your storytime together and, as always, fertilize