Your shown example of Scott 504, is in a vertical "strip of 3". Stamps are often available in multiples, such as a pair, block of 4, etc. Even as a minimal value stamp, multiples often are more desirable to collectors. This particular strip of 3 is still of small monetary value.
Sometimes stamp multiples are useful to identify plate position, potential plate flaws that are known in certain positions in a pane, or for some issues, scarcer and not often found in multiples.