Your example was the standard notation for vocal parts up to about 1950. The beams indicate the notes sung to one syllable of the lyrics. You will find almost all "pre-computer-engraving" vocal scores written that way.
The slurs in your example show exactly the same thing as the beaming, and were sometimes omitted, except over quarter notes or longer which don't have beams.
For long passages (i.e. more than one bar) sung to a single syllable, the beaming followed the normal non-vocal convention, and a slur indicated the extent of the syllable.
In modern notation the beaming follows the same rules as for non-vocal music, and slurs are used to show groups of notes sung to one syllable.
When all music was printed from metal plates engraved by hand, it was no more time-consuming to produce this style of beams. Computer notation software has more or less killed it in favour of beaming all parts the same way, since the old style vocal beaming can't easily be automated.
Singers seem to be divided about which convention is "best" - some find the modern beaming convention hard to read because it hides the rhythmic connection between the words and the music, others (like you) find the old-style rhythms hard to decipher.
Incidentally your example shows another common feature of editions from that period, namely that long notes tend to appear in the middle of their sounding duration, not at the start as in modern music engraving. See the half notes in the first and last bars of the bass line, for example.