Having a Spider-Man game to play at the launch of the next console is quite attractive to many.
The thing to remember that for any new console, a good chunk of the target will be people for whom the PS5 is going to be their first console ever (teens, parents of teens who want to give their kid the top-of-the-line thing, people who were saving money to buy a PS4 but then when they learned that the next one is coming, saving to buy the PS5 instead) so a Spider-Man game validates their pre-meditated choice and doubles down on it. Spider-Man and Miles Morales especially became big with ITSV, especially on the target demographic and that movie is still fresh in everyone's minds, so in some sense playing as Miles on PS5 fills the gap before the sequel comes.