The simple answer is when it is one of an unbroken chain of eighths in a Bach Cello Suite, as in the Courante of the C major. Some of these eighths are articulated melodic values while others group into non-metric larger composites--arpeggios, for example, or melodic slides. The differences are not always clear, nor do they need to be.
That is why there are repeat signs: you get to do it twice, each time with a different twist on it.
When played strictly, without reference to shifting melodic and decorative purposes, the only thing that comes across is the bar line--clearly an error in Baroque music.
