Napoleon considered honour the chief incentive of french social system. (and yes, he added Dantzig chocolates for the top ranks).
To give titles was giving honour to new individuals, and at the same time remind the old aristocracy that merit superseded old name.
To make them hereditary was an even more powerfull incentive, the individual knew that he will transmit is glory to his heirs - but not his social position.
Even feeling superior due to their title, these heirs will be challenged by new individuals, and will have to proove themself.
Relatives of marshals had accelerated career, being AdC of the head of family. But still, they would not get command position due to their name.
Son of Murat was the youngest Lieutenant de Vaisseau of french navy. But no more.
Traditional monarchy has a radically different functioning: due to his name, an old emigrant who did not navigate for twenty years could get command of a fine frigate and ground it by fine wheather (La Meduse). Although dozen of seasoned captains were available.