Of course it's not necessary if both houses pass identical legislation, except that almost never happens. Remember the healthcare reform bill and the twists the Dems had to do to reconcile the two bills.
If the bills are not identical word for word, then a conference must be held. And of course the Constitution does not specifically mention conference committees. It DOES say that each house must pass and sign the exact same bill before it can be sent to the president.
The conference committee is the method devised to do just that.