These capture moments in time, are easy to read (especially when pushed for time, though it doesn't make them easier or quicker to write), and good ones stay with you long after you've read them. They're a good way of testing out whether you like a writer's style or not. They cover all genres and tastes. They can be a great way of coaxing a reluctant reader to read at all (and hopefully go on to read more, perhaps going on to novels etc).
These give the space to develop characters much further. I loved being able to reveal more of my main characters in The Trouble With Mother. It made the characters more real for me - and hopefully will for readers. I'm hoping to bring TTWM out in some format and will post more on this later. Novels can show an entire universe (think The Lord of the Rings or the Harry Potter or the Discworld series just to name some examples). You can also have sub-plots which add depth to the overall story. As a reader, I love coming back to where I'd left off from my last read and continuing the journey...
These capture specific moments and can appeal directly to the emotions. I always feel goosebumps when hearing anything by the war poets. (I recommend Radio 4's Poetry Please with Roger McGough if you want to expand your knowledge of poetry. It's certainly expanded my knowledge and given me a greater feel for what I like). Sometimes a poem can say something that wouldn't come across as well in any other form.
I've always had soft spot for these. They're also good for an instant laugh, a kind of literary pick-me-up. I don't know why some look down on limericks. A good limerick will need as much crafting as a good poem. (Just because something is shorter in length doesn't mean you take less time to get it right).
Can deal with fiction or non-fiction. I'm glad to write both, here on my websites and on Chandlers Ford Today. Can bring in good feedback and help you connect with readers, which is never a bad thing (unless you're the hero of Stephen King's Misery but that's another story!).