Ever become aware of a repeating sentence structure while reading? I have. Once I notice a repeating rhythm, I have trouble ignoring it, and it distracts me from what I'm reading. Since most writers want their readers focused on their story, not their sentence layout, variety is a good idea.
By far, the worst offender I've found while editing manuscripts--and sometimes while reading published novels--is the repeated use of introductory phrases. There's nothing bad about using introductory phrases. My point is that they shouldn't be over-used.
Taking care not to wake her young son, Mary tiptoed across the room and closed his window. As she returned to the doorway, Billy stirred in bed and moaned. Sensing he was having a bad dream, she stood beside his bed. Billy mumbled a name Mary couldn't understand. Leaning closer, she listened.
Even with a simple, direct sentence thrown into the mix, notice how the rhythm repeats? The effect gets more noticeable after several paragraphs until it becomes sing-songy and distracting.
Sometimes writers get into the habit of tacking a phrase or two on to each sentence.
Mary tiptoed across the room to close her young son's window, taking care not to wake him. Billy made noises, thrashing and mumbling in his sleep. Mary stood beside his bed, sensing he was having a bad dream. Billy mumbled a name she couldn't understand. She leaned closer, hoping he'd repeat it.
Again, readers begin to expect this pattern to continue, and it becomes sing-songy.
Repeating sentence structures are hard to catch in your own writing, but it's important to try. In addition to the ones mentioned above, look for successive simple sentences or runs of compound sentences of about the same length. I urge writers not to worry about varying sentence structures as they write first draft, but keep it in mind during edits. We all have our favorite sentence structures, things that work well for us. Using favored structures can help define our writing style. Over-using them can distract our readers. Mix it up.