One of things that the declaration of the Immaculate Conception did was end the centuries old debate. Sts Bernard, Bonaventure and Thomas Aquinas were on the losing side but so too were those who claimed like Aristotle and many Jews and Muslims that the humanising or ensoulment of a person happened sometime after conception. The Immaculate Conception is a pro-Life doctrine, Orthodox who believe that Mary is Immaculate but not from the moment of conception tend to believe in a later or progressive ensoulment, which is presumably why the Orthodox Christians are not as committed to Pro-Life issues as Catholics or Evangelicals.
I can't help, being a little cynical thinking that the infallible definition of the Immaculate Conception was a bit of Papal muscle flexing, simply because the doctrine was not believed "always and everywhere" but it is also an illustration part of Divine providentiality that it states that for the Blessed Virgin, and therefore the rest of us, that she became human from the moment of coneption. It was not so important in the 19th century but of great importance in the 20th and 21st century.