For example, a conductor can tell their musicians to resume at bar 387, so that the musicians have to find the nearest bar number in their parts (e.g. 385 or 390) and count back or forward a couple of measures. Even if the number 387 is written at the appropriate bar, it might not particularly stand out. But if there is, for example, a big, bold letter M in the score and parts, it is much easier for the conductor to just say "begin at letter M". Even if the conductor were to say "one bar before letter M", that would still be more convenient than saying "bar 386". Alternatively the conductor could first say "before M..." and allow the players time to find M and then say "one bar".