The reason why rehabilitating the image of Stalin is important has less to do with Stalin himself (who was as human as anyone else and made mistakes) and more with what he and his period in the USSR and the world as a whole stood for.
By denouncing Stalin, Khrushchev did not just denounce a person, he denounced an entire system, which is the system of socialism that had been built up in the USSR up to that point, even as he pretended that he was not attacking socialism itself.
The ideological foundation of the country was fatally undermined, the population demoralized and ideologically confused, revolutionary enthusiasm crushed by the repressions of the "Stalinists" (aka principled Marxist-Leninists), and the resulting historical nihilism led to the tragic and catastrophic consequences of the Gorbachev betrayal and counter-revolution.
By comparison, China avoided making the same mistake, even though, arguably, Mao made just as many or even more (or more severe) mistakes than Stalin ever did. Yet even as new party cadres came to power in the CPC who would have had much more legitimate reasons to hold grudges against Mao, they did not do the same as the Khrushchev clique did to Stalin.
Instead Deng Xiaoping and his successors prioritized the interests of the revolution and of China and refused to throw out this huge part of the ideological foundation and popular legitimacy of their revolution and their socialist system, so much that it has become a part of national identity regardless of how critical individually they may be of Mao or specific policies of his.
In addition to this, Stalin was also an enormous global symbol of anti-fascist resistance. By attacking and slandering Stalin the whole world communist movement was throwing into a tailspin from which it has even to this day not recovered. It also opened up the door for the rehabilitation of fascists and fascist collaborators, even in the Soviet Union itself.
In connection with so-called "de-Stalinization", Khrushchev gave an almost blanket amnesty to "political prisoners" and released countless fascists and fascist collaborators, especially in Ukraine, who then proceeded to worm their way into positions of power in the USSR, or to emigrate to the West and build up emigrée organizations dedicated to glorifying Nazi collaboration and to the destruction of socialism that would eventually return, to places like Ukraine and the Baltics, and turn these societies increasingly fascist.
The rehabilitation of Stalin is a rehabilitation of socialism, proletarian democracy and anti-fascism, a rehabilitation of the revolutionary legacy of the Soviet Union when it performed some of the most impressive feats of any society in human history, of building up and defending an industrial and military superpower led for the first time ever by the working masses and not by an oligarchic, aristocratic or financial elite.
"Ok we went a bit hard on the sparrows, how about you go beat your dad with a stick now "