Another use of conditonal assembly it to permit build of two different product configurations from the same source code...one low end version with some features not assembled in that you sell cheaper...and a high end version with all features present so you can charge full price.

This technigue can be valuable as opposed to using jumpers to select product features because the low end code may link to a size to fit in say 16K flash whereas the high end may link to a size above 16K where you would use a part with 32K flash capacity. Most likely the processor version with the 16K flash would be less costly than the one with 32K and so thus the low cost product version has the additional advantage of a lower cost processor.

Michael Karas