As to Hit Dice, they're mainly there due to a number of spells affecting a set number of Hit Dice.
In the Monster Manual it typically lists things like:
Hit Dice: 5d8 (22).
So they're 5 Hit Dice monsters, and have 22 HP.
I mean normally every level except first you're supposed to roll your characters HP. They decided to go with "max HP at first, average HP after 1st" which has been a common house rule for a long time. They might have even made it a "standard" option for 5e.