I'll comment a bit further on the purpose for holding the note down. It's more common to see it in the bass (it's called a "pedal point") because holding certain bass strings open while playing higher notes creates a richness of tone. Debussy liked to do it often. Have a look at "Isle Joyeuse", between 3:00 and 4:20, where he uses it extensively:
Look at the arpeggiated figure at 2:46 (where it says "A Tempo"), and then compare that with the similar figure at 2:54, which has the double pedal point at G#-D#. You'll probably notice that there's a richer and more complex tone, even after the pedal point notes die away. There are several more examples that you can compare further along in the passage.
By the way, this is one bit of music that makes extensive use of the sostenuto (middle) pedal.