Used in higher limit games. Above the low-limit games, the limits might be 5-10, 10-20 then 20-40.
In the 20-40 game, rather than use a bunch of $5 chips, or a mix of $5's and $25's, its easier to use these $20's. Also faster.

In a 20-40 limit game, you would be betting 4 x $5 chips before, and 8 x $5 chips after. It's slower, and you don't have miscounts (7 or 9 chips, rather than 8). And if there are raises, also easier to calculate the next increment. Much easier to run the game with $20's ... plus smaller chips for whatever the antes or blinds are.

Finally, one $20 chip is cheaper to buy than 4 x $5 chips. Not a big concern, but also a factor.