Subordinate bar numbers are useful for numbering repeat endings, and for situations when the music is being altered, but the original bar numbers cannot be changed.
For example, you can use subordinate bar numbers to show where music has been added if a previous, shorter version has already been rehearsed. In this situation, players have likely started to associate certain parts of the piece with particular bar numbers, so if four bars need to be added after bar 10, they would be numbered 10a to 10d. The bar number of the following bar then continues from 11 exactly as it did before the new bars were added.
They might also be useful if you want different bar numbers for a repeat ending.
Subordinate bar numbers are shown with lowercase letters by default, but you can present them as either uppercase or lowercase letters.
You can show both primary bar numbers and subordinate letters or only subordinate alphabetical letters in subordinate bar number sequences.