Adding a spacer to a stock motor raises the head, and by doing so lowers the compression. Being as you can make 330hp on a stock bottom end its a cheaper alternative to building the engine and still make some decent power.
Most aftermarket Pistons require a base spacer because the ring lands are thicker. Doing so raises the face of the piston making it taller. So to compensate you raise the head to get your clearances back in check. This is for gen 1 bikes.
Gen 2 bikes don't require a base spacer for most turbo Pistons because the geometry is a little different on those Pistons to make up for the 2mm of extra stroke from the crank.
Why not use a thick head gasket? Glad you asked. The thicker the head gasket, the more prone they are to move around from combustion pressure. The more surface area you expose to combustion pressures, the easier it is to move and blow out head gaskets.
We're lucky suzuki uses a "modular" engine where the cylinders are separate from the upper case unlike a lot of the kawis. Those you're forced to use thicker head gaskets depending on the combination to keep your clearances. Also put you in a bind with stroker cranks with shorter rods. Since they don't have the luxury of moving deck height around they have to deal with extreme rod angles which wear bearing quicker, and tend to break rods, or spin bearings if you're lucky.
