What are these that I get for achievements? Do they serve any purpose?

Berath's Blessings? They give you extra perks if you start another playthrough- extra money, better equipment, bonus stats, etc

What are these that I get for achievements? Do they serve any purpose?

 

They are for later playthroughs. Simply quality-of-life improvements. Nothing more.

Something funny about this, by the way: you get these blessings in Deadfire, which is already very easy, but not in, say, Kingmaker, which is pretty damn hard. It never rains but pours, it never shines but scorches.