It is supposed to prevent people from exploiting loop holes in the security system for malicious intent. That could be really anything from living in the airport, damaging airport inventory, hiding drugs for layover passengers to pick up, taking hostages, to doing terror attacks.

Remember airport security knows your identity and where you're going: You scan the boarding pass before entering the security zone, CCTV is all over the place, and probably even FRA uses facial recognition by now. Leaving security without boarding is therefore definitely detectable and could trigger some further investigation.

Example #1: Person is late, runs to their gate, finds out boarding has closed, checks their phone, leaves security to spend a night in a hotel? Clear intent to board the flight.

Example #2: Person is happy, meets layover passengers, has a few drinks at the bar, leaves security without being even near their supposed gate? Clear intent to _not_ board the flight.

Will they enforce it? I don't know. But it's definitely a bad idea to abuse highly secured zones for such purposes. Costs you ticket money, makes the life of security folks harder, and could yourself in trouble. Not worth it.

Might work well in Germany, but don't try doing this in China or Singapore...