Why was it necessary to assemble all the people and for them to lean their hands on the Leviyim?
Rashi: Since the Leviyim were being given as a Korban Kaparah instead of them, it was fitting for them to stand by their Korban and to lean their hands on it.
Rashi writes that he assembled all the people to stand by their Korban, for the Leviyim were as a Korban in place of them. They were only in place of the Bechoros!
Moshav Zekenim: The verse often speaks like this (as if it discusses everyone, even though it does not)