Rhythmic sentences, whether they are long or short, help people remember things, as Suzanne L. Medina has noted. This is because the rhythm of a sentence gives your brain something to latch onto aside from the words and their meaning. This extra hook helps readers or listeners to remember more of a sentence because it provides them with something to link to that sentence's content aside from its meaning. This makes long sentences a great tool for memorizing a long list of ideas or a single complex idea.
Although modern writing favors shorter, simpler sentences, an essay, story, or description that consists entirely of short sentences can make for dull reading (Reference 2). Long sentences can help to build variety, and if they are also rhythmical then they will be a pleasure to read. Long rhythmic sentences can also build a reader's or listener's appreciation for what has been going on in previous sentences and to energize him or her for what will be going on in later sentences. In fact, if used well, long rhythmic sentences can be a powerful means of keeping a reader or listener interested in a piece of writing by adding variety and by showing how key ideas fit together more clearly than a long section of shorter sentences.
Long rhythmic sentences go much further to draw your reader into the atmosphere of your writing as you describe it than short sentences, often because long sentences simply convey more information. Such sentences have this kind of drawing power because they can entice the reader to march through them, tiptoe through them or rush through them depending on the rhythm that is used. Further, these sentences' length gives readers or listeners enough time to get the atmosphere of a setting made entirely clear in their minds.
Along with the above effects, long rhythmic sentences also have the power to lull readers or listeners into their pattern. This power can work to get readers or listeners to stop paying attention to what the sentence is saying but it can also be used to provide readers with a patterned input that a writer or speaker can use to give them important information. This power is, in fact, what gives these sentences their mnemonic and atmospheric power as it provides readers or listeners something in which to base their memorization or interpretation of a sentence's contents. Of course, this effect of long rhythmic sentences is best used if these sentences are placed in a good mix of shorter sentences to offset their lulling effect.