Matt Whiting wrote in newsj6pj.130$kD5.1392
@news1.epix.net:

Ricky wrote:
After reading more on this I have found that the German's were very
concerned with the ability of their aircraft to get above the enemy as
quickly as possible. An attack from above (especially from out of the
sun), was found to be an extremely effective method of victory. The
amount of lift generated from 3 wings was found to enhance climb
performance quite significantly, thus affording German pilots the
abilty to attack from above as was desired.

I really doubt that was the reason as lift can easily be increased in a
number of ways other than adding wings. I think structural strength was
the primary reason for more wings in that era.

Not in this case. One of the advantages of a bipe is the rigging allows an
extremely rigid structure with light weight and an ability to have a wing
that is unrestrained by the need to conceal a lot of structure ( thick
spar). The DR1 was revolutionary in that all the panels were canitlever.
There was no external bracing except the roll wires between the cabane. The
interplane struts were redundant.

Bertie