Why is the significance of the fact that Hashem forbade Aharon and his sons to refrain from having a haircut?
Ramban #2, Rashbam and Seforno: A Kohen Gadol is anyway forbidden to show any sign of mourning, 4 and the Pasuk is coming to extend the prohibition to Elazar and Isamar, who had the status of Kohanim Gedolim, since they had been anointed with the Shemen ha'Mishchah. 5
The same applies to the prohibition against tearing their clothes (Torah Temimah, citing Mo'ed Katan, 15a).
See Ramban and Sifsei Chachamim.
However, this D'rashah is only an Asmachta (a support from a Pasuk, since the prohibition regarding an Aveil having a haircut and leaving is clothes un-torn is only mi'de'Rabbanan [See Ramban]).
As the Torah explains in Pasuk 7 (Ramban, Rashbam and Seforno).
Why does the Torah see fit to add "ve'Lo Samusu"? Is it not obvious?