In a perfect world, your wheel/tire combo would always put the most rubber on the road for optimal traction and handling, have the flush stance that makes you look over your shoulder when walking away from your ride, never hit the frame or fenders, and allow clearance for brake mods. Let us know when you find this mythical Shangri-La. In the real world, we have to roll with what we have, including wheels that may not have the right offset or bolt pattern.
Wheel spacers position the wheels further away from the hubs. This can place the tires closer to the edge of the fenders to gain that desirable “flush stance”, and widen the track, which also increases stability for better handling. Installing wheel spacers can also provide clearance for wider tires and allow the installation of aftermarket big brake kits. Adapters both space the wheels out and allow wheels with a different bolt pattern to be mounted.