From the Guardian:
“Bach decides he’s going to enrich the sound of the instrument by tuning it down, taking the A string down to a G,” explains cellist Yo-Yo Ma. “He’s saying, ‘If I do that, I can get more overtone, I can make the chords richer, make it more polyphonic.’ He’s trying to make a single-line instrument give the illusion of several voices.”
Does that make it more difficult to play? He sighs. “Gosh, yes. It makes it more impossible.”