The KISS prescription is only an admonition to writers to aim for brevity when they write. It’s actually not a prescription to use simple sentences instead of complex, compound, or complex-compound ones. In English, as I explain in my book Give Your English the Winning Edge, “simple sentences are not necessarily short or uncomplicated, and compound or complex sentences are not necessarily longer or more complicated than simple sentences. This is because the structural complexity of a sentence doesn’t really depend on how many words it has or on how many phrases are attached to it, but on how many clauses and what kinds of clauses are to be found in it.”
(Recall now that in a sentence, it is the number of clauses—those groups of words that contain a subject and predicate and that typically can function as a complete sentence by themselves—that determines sentence structure, not the number of words or phrases a sentence has.)
Below, for instance, is a 20-word sentence of simple structure—in short, a simple sentence—that’s definitely not short and uncomplicated because aside from its main and only clause (“the woman wore a starkly red dress and red high-heels”), it also carries an absolute phrase (“decided to upstage everybody”) and a prepositional phrase (“at the costume party last night”):
“Decided to upstage everybody, the woman wore a starkly red dress and red high-heels at the costume party last night.”
In contrast, here’s a complex sentence with only nine words: “He who was declared winner doesn’t deserve his win.” The main clause is, of course, “he doesn’t deserve his win,” and the subordinate clause, the relative modifying clause “who was declared winner.”
So, you ask, how then will writers know when it is advisable—not necessarily “better,” which is how you worded it in your question—to write a complex sentence instead of a simple one? It’s advisable to do so when the writer wants a wider opportunity to clarify ideas or establish their context better within the same sentence framework; that is, to elaborate on or texture those ideas without coming up with a new sentence or several more of it.
Take a look at the following complex sentence:
“When the general manager returns from his foreign trip this Sunday, meet him at the airport unless you get a call from me by seven that morning not to do so.”
In the sentence above, the independent clause “meet him at the airport” is flanked by two subordinate clauses: “when the general manager returns from his foreign trip this Sunday” and “unless you get a call from me by seven that morning not to do so.” This way, without starting a new sentence, the writer is able to provide the statement in the main clause, “meet him at the airport,” both its motivation and its limitation.
See how that statement would look like and hear how it would sound using only simple sentences:
“The general manager returns from his foreign trip this Sunday. Meet him at the airport. Do so by seven that morning. I’ll call you if you don’t have to.”
The simple sentences in the statement above follow the KISS rule, but the speaker would certainly sound unpleasantly simplistic by talking that way. It’s not the natural way for people to talk. Even in ordinary discourse like this, in fact, people tend to use complex sentences without even becoming conscious that they do.
That, as simply as I can explain it, is the advantage of using complex sentences instead of simple ones. I must also add in closing that there’s no poetic license involved when choosing complex sentences over simple ones; it’s simply in the nature of language to use complex sentences every now and then to texture ideas with minimum effort.
