By law in germany (can be a bit diff based on your contract) you are supposed to tell your employer if you are sick upto 3 days. More than 3, and you need a sick note from a doctor.

As for why, the company needs to record sick days and also might need to submit it to tax authorities if asked. The company would get some tax back if an employee is sick. There are also cases where the health insurance might pay the company if the employee is sick.