With the zetsu clone he absorbed, many people think the reason was to let him use mukoton. But since the zetsu is actually weak as a fighter, this is not likely the case.
But, a zetsu clone is, a crap load of Hahirama cells. Which also means~ 2nd son body. We all know that the 1st son got the eyes, and the 2nd got the powerful body.
With this powerful body, Orochimaru can finally complete his sage mode. All he needed was a body that can withstand that ability. (Here)