Where I come from (Argentina), it is normal to have drains in the bathroom, the kitchen and the laundry room. I have encountered the same situation in modern homes in many other countries. Particularly bathrooms and kitchen are places where bacteria grows happily and strong due to organic residues and humid, warm air. For this reason, the floors (and every now and then the walls) are cleaned with bleach (with plenty of water), which is pushed to the drain.

Modern German buildings spare costs by not installing the extra siphons for the extra drains, assuming that people do not use water at all for cleaning. Actually, many German people I know just use the vacuum cleaner and maybe a wet rag for some visible spots. Not very hygienic, I must say... but then again, their cooking is usually limited to moving things from the freezer to the oven. In any case, there is no real downside for the extra drain besides the (relatively low) extra costs.