Found a useful thought on the page https://wol.jw.org/en/wol/d/r1/lp-e/2004206#h=23:0-24:0, which notes:
How was Moses made “God to Pharaoh”? Moses was given divine power and authority over Pharaoh. Hence, there was no need to be afraid of that king.
Highlighted here on another page https://wol.jw.org/en/wol/d/r1/lp-e/2002564#h=27:0-28:0
is the following idea:
Aaron served as a prophet, not in the sense of foretelling future events, but by becoming Moses’ spokesman, or mouthpiece.
Consider that the Pharaoh was himself seen as a god, and that Moses acted in behalf of the "God of gods", who "removes kings and sets up kings" (Daniel 2:21,47). It must have been a fearful thing to be at the Pharaoh's mercy, as the Egyptians would probably have obeyed the Pharaoh without any questions asked. As consequence, it may have been the first time for someone to speak against the Pharaoh at all, not to mention the bold and open manner in which it was to happen. No wonder then that Moses could do with all the reassurance he could get!