Well, they're not really tab stops in the same way as the rest because once
you set them the bars appear without your having to insert a tab character.
I'm told they were originally intended to allow users to create table
borders before table functionality was included in Word. They're now
evidently regarded as an obsolete legacy included only for backward
compatibility (and perhaps of some use in creating text files).
Each user will have to find his own use for them, but here's what they do:
they create a vertical line the height of the paragraph (not just the height
of the text but also including font leading, line spacing, and any Space
Before/After), which means that, when used in several contiguous paragraphs,
they create a continuous vertical line. Used in combination with underlines,
they can be used to create a grid effect similar to table borders. Sometimes
this can be useful in situations where a table is problematic (or more
trouble than it's worth).
