In addition to the points that Manu makes, with some minimal calculations, if one knows the ABV in his boiler, and where his cuts will be made, one can have a final product ready for casking, bottling, or other post distillation procedures without having to resort to dilution with water. Dilution adds another step that requires control and can influence final taste and quality. It can also cause some fatty esters to precipitate out of the distillate. Having the right ABV out of the condenser takes this step out of the process and removes another opportunity for quality to differ from the desired goal.
Jack