I don't know why the quavers in the bass are beamed across barlines the way they are. Does it signify phrasing or is there another reason for it?
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Yes, it’s a way to signify another level of phrase grouping. Just as your beaming within a bar can indicate groupings of beamed notes beneath the level indicated by the slur, this shows that the composer or editor wanted to indicate this grouping that extends across bar lines.
The beaming itself is already a kind of editorial decision because each quaver has a rest in between, so they could have been notated with flags. But flags plus two levels of slurs for phrasing may have been seen as less readable than using beams, since the symbology for each level of phrasing would be the same instead of different, and by using beams we also satisfy conveying the rhythmic value at the same time.
Looking at the phrase mark itself, it would appear that it is because of phrasing. Although it's not that clear where the phrase mark in bar 3 goes, being at the end of a line. I guess it goes on to be a phrase over 4 bars - bar 3 to the start of bar 6.