Generally, this is considered grammatically incorrect, and called a stylistic error. There are many examples of run-on sentences used as literary devices in literature. However, not all long sentences are run-on sentences, for it is quite acceptable to combine different related ideas into a compound sentence. Nevertheless, without using appropriate punctuation rules, a compound sentence becomes a run-on sentence.
How are they used as a literary device? What can you do with them? Unlike other literary devices like metaphors and allegory, it doesn't seem like run-on sentences can do much.