Pre-AIA, when it was "first to invent", there needed to be a scheme to figure out who won that contest. It was essentially, first to conceive as long as the first conceiver proceeded diligently from conception to reduction to practice. Person A might reduce to practice (which can be accomplished by filing an enabling application) first, but lose to person B who filed later, but who conceived earlier. But if person B conceived before A and could prove diligent effort towards reduction to practice from a date before A's conception, B would be the first to invent. B not working on it during a one week vacation might knock out person B's chances.
Also, someone actually reducing it to practice would not make it prior art leading to a 102(a) rejection for someone else as long as the invention was kept confidential and not offered for sale.