The impetus for cremation likely came from Hitler himself. Testimony from Speer and other survivors of the Bunker's last days mention that Hitler spoke not only of suicide, but also preventing his body from being displayed by the Soviet enemy. News of how Mussolini had his corpse hung in Milan for jeering and ridicule further added to Hitler's desire to have no recognizable corpse left for the Allies. Hitler detailed his his personal adjutant SS-Sturmbannführer Otto Günsche to the cremation duties and the dictator noted that he did not want his corpse to be put on display in Moscow.
In the end, the cremation was part of Hitler's final attempt to control the narrative of his life. Rather than face up to the consequences of his lost war, he instead opted to choose his own end in a way to retain something of his mystique. While the cremation was a shabby affair and partially successful, it did deny the Allies a viable trophy and fueled cracked conspiracies that he somehow escaped in 1945.