Yes. Hook your powerheads up to it.
How many powerheads you runnin' in that 90. Should be at least 2. 3 Wouldn't hurt.
It makes the powerheads do the alternate flow thing. Off and on at differnt intervals. Helps for a more random flow instead of things blowing the same way all the time. Simulates a more natural reef where things blow/flow different directions.
Very good addition to a tank IMO
And the first time you push the Night button (next to the feeding button), it'll remember and repeat that every nite at the same time to simulate more subtle nightime flow so everybody can rest better.