Picture a piece for 3 players -- 2 soprano instruments and Basso Continuo (a bass instrument + let's say, harpsichord). It's a piece with a recurring harmonic structure; say, Passamezzo or ciacona -- think Pachelbel Canon.

The BC in this particular instance might only have barlines every 8 measures. The soprano parts might have barlines every 8 measures too, but might also have them where a significant change to their melodies change. Like, when a flurry of 16th notes begin to crop up.

Now, this isn't always the case -- it's definitely inconsistent, but such things show up enough that those hints cannot be discounted at all, and seem to be enough of a performance hint to at least be noted and appreciated by the performer.