It always fits together with the other LEGO blocks. This is exactly what is happening in normal home building. Every piece is fitted to size. An other advantage of standard dimensions is that manufacturers can produce elements, not knowing what the building will be in which it is fitted. This opens the way to mass production and thus lowers prices.
Mass production asks for limiting the amount of available dimensions.
On the other side, standardized dimensions can be a harness for the design. A standard dimension could be just a little too long or too short for your goal. So you should have enough available dimensions to reach your goal. System A is a fractal modular preference dividing line dimension system with fixed joint width . This asks for some explanation.