When sailing at a constant heading, not all the water under the board is moving in a straight line aft. At the location of the fins there is a transverse component present in the streamlines as you can see in the following figure
The fins should be aligned with the flow when travelling straight without any drift angle. Like when you are being pulled by a rope. When the fins are properly aligned with the flow, the fins will not generate any lift and therefore have minimal drag.
When the fins are not properly aligned with the flow, for example when you have 0 degree toe-in for the side fins. Both side fins will generate lift towards the rail, this transverse component of the lift will cancel out. By generating lift you will generate drag on both fins and the board becomes sluggish.
In this example Θ is the toe-in angle; here you can download an XLS-sheet to help you calculate the toe-in angle. Typical toe-in values in a thruster setup are be between 1.5 - 2.5 degrees.