Primal Legendary items are the strongest Legendary items in the game, so you should definitely pick them up and see if they are useful to you.
Any character in the same league and season can get the drops even if that character has not beaten GR70 themselves. Once you beat a Greater Rift rank 70 you will unlock the chance for any Legendary item to drop as a Primal Ancient Legendary instead, with a very low drop chance. This means that you will only unlock it for the characters in the same season and league (hardcore or softcore) as the character you beat GR70 on.
If you want to get Primal Legendary you need to beat a Great Rift Rank 70 solo on any character on that ladder.
Primal Ancient Legendary items in Diablo 3 is a type of Legendary item that is even better than the Ancient Legendary items.
