Many have the minhag to say the hagadah (from עבדים היינו until לכפר על כל עוונותינו) sometime Shabbos HaGadol afternoon (this is found in the Rema, in Shulchan Aruch), because the actual ge’ulah began on that Shabbos (10 Nissan). That is the reason the Mishneh Berurah gives.
Others say that it is to make the text of the hagadah somewhat familiar, as we find sometimes in Halacha (to prepare an unfamiliar text beforehand).
The Gra says not to say it. The Chasam Sofer as well was against saying it.
(On a personal note, the Rav started saying it again, after a hiatus of quite a few years.)