Thinner picks are often needed for narrow keyways or high/low pin stacks.
They also work best with top of keyway tension.
These 0.020 picks allow for slightly more working room than a standard pick.
This extra room can allow for different angles of attack on the pins resulting in more picking possibilities.
At .020 the picks lie between the traditional stiffness of a standard pick and the flexibility of a .015

Standard picks are 0.025 thick and their is nothing wrong with flexibility in a .015 pick.
Often you want that flex to help position the pick.