The legs were the part of Arich Anpin in which the vessels were rooted during the time of their descent. There is nothing lower than the legs, and during the time of their descent, the vessels were rooted only in such a lowly part because of their own lowly status.
Then afterwards (as a result of the folding of the legs) they also became rooted in Chessed-Gevurah-Tiferet as the extent of their repair increased. This was what was gained from the folding of the legs – that the vessels also became rooted in Chessed-Gevurah-Tiferet of Arich Anpin, as explained in the writings of the ARI (Etz Chayim, Shaar Zeir Anpin ch. 2).