Studies show that your concentration remains sharper if your visual processing system is jarred out of its complacency at frequent, semi-random intervals, preventing you from lapsing into a low-effort 'skim-reading' mindset. It's something to do with the predictive processing model; over time, your brain comes to rely increasingly on top-down processing if its expectations are consistently met, and I guess there is some interplay between visual processing and semantic comprehension in this context.

(Just kidding! I think it is simply an annoying affectation.)