If you were working for them like a regular employee, that has quite a few pitfalls - you'd need to perform the duties that an employer has then (get a "Betriebsnummer" (employer registration number), register your contract etc) - working "on a freelance basis" where you actually act as an employee isn't legal (https://www.existenzgruender.de/SharedDocs/BMWi-Expertenforu...). And if compliance is important for a company, they'd need at least someone that is familiar with labor law in a country before they start employing people there, even if they don't explicitly need a business entity.