Why was this necessary? Whatever ideological impact on Mitzrayim that the Makkos may have achieved was temporary and forgotten, having no lasting effect. Although we find that many non-Jews converted to Judaism during the Persian and Roman conquests, we have no such record of Egyptians embracing Judaism while Egypt dominated the Hebrews, nor did monotheism replace the idolatrous ways of Mitzrayim. Whatever belief in Hashem on the part of the Mitzrim (Egyptians) that may have been attained at the time of the Makkos obviously dissipated and did not last long. Why, then, did Hashem proclaim that the goal of the Plagues was for Mitzrayim to recognize and admit to His greatness, if such recognition and admission were to be worthless in the long term?
There is an halachic principal that governs deliberations of a beis din - a rabbinic court - which states that the younger, less seasoned members of a beis din must put forth their arguments in a case before the more senior and expert dayanim (judges) do so, in order that the younger and less seasoned members not be inhibited from expressing their opinions, according to some interpretations. (See Sanhedrin 32A and 36A.) One way to view this principle is through the lens of human psychology, such that one who feels that another is superior to himself will give more weight to the other's views and not be of an objective mindset, even if he has every reason to be confident in his own position and attitude. In order that the dayanim of lesser stature evaluate the case objectively and not subconsciously adopt the approaches of the senior judges, to whose views they would naturally and instinctively defer, these less seasoned dayanim must present their analyses first.
The Makkos, Yetzias Mitzrayim, Mattan Torah (the Giving of the Torah) and the entire experience in the Desert were designed to instill emunah (faith) and commitment in the nascent Jewish nation, and all that could be done to maximize this goal was done. The Jews were slaves and had been trained for generations to view the Egyptians as superior to themselves. Therefore, it was not sufficient for the Jewish nation to merely witness the supernatural afflictions that befell the Mitzrim in order to be inspired to the fullest degree of belief and commitment to God. Rather, Hashem sought for Pharaoh and his people themselves to openly admit to His might and ultimate mastery over all, for when one sees that another whom he feels is superior - especially if that other normally negates the issue at hand - suddenly totally endorses and embraces it with unparalleled conviction, the impact on the first party is quite potent. Just as the less experienced member of the beis din may be swayed by the opinion of the senior member more than by his own strong convictions, in an even greater manner were the Jewish slaves struck with belief in Hashem when witnessing the idolatrous and superior Egyptians profess unconditional and absolute belief in Him and His omnipotence.
This is why - for the ultimate sake of the Jews themselves - God dictated that Pharaoh and his nation must openly admit to His wonders. May we again experience such wonders soon.