Well in Germany specifically it's a legal issue (and some other EU countries), because there are labour market laws stipulating the maximum amount of time someone can work per week (48hrs).
Sure, you can fly under the radar if you're working on your own business/start-up by simply lying about the hours worked, but if you're doing paid work for another employer it will be obvious, and they would not be legally able to employ you if total working time goes above 48hrs/week.
Sure, you can fly under the radar if you're working on your own business/start-up by simply lying about the hours worked, but if you're doing paid work for another employer it will be obvious, and they would not be legally able to employ you if total working time goes above 48hrs/week.