I am not certain but I suspect that it is simply a decorative practice. I believe I have seen it done on older, non-Jewish books and I assume that the practice has faded in favor of more economical/contemporary styles. Jews who buy seforim, on the other hand, are a little more inclined for "classic" styles and or more interested in a more distinguished graphic design (Goldleaf is not common among general books anymore but the rule among seforim).
Additionally, while I recall there being basis for leniency, opening and closing a book with words stamped on the side (such as the owner's name) is an old problem mentioned in the poskim and having such decoration (which lacks any coherent figure) decorates the side while preventing such stamping.