I plan to fly with German citizenship from Spain to the US - San Francisco. I got already an ESTA 
I read in every source that you need an backflight for getting a ESTA. I filled out the form for the ESTA for some while ago without any return tickets. I am not sure if they wrote something like "at the airport you need return tickets to show". Do I need return tickets or is it just optional?
Yes, to enter the United States using the Visa Waiver Program you must have an onward ticket out of the US (technically out of North America).
Before you fly, it will be checked by airline check-in that you have a valid ESTA authorization and an onward ticket. If they can't verify automatically they will ask for proof of your onward flight. If you don't have an ESTA and onward ticket you won't be allowed to board the flight.
After your flight, when you are interviewed at the US Immigration checkpoint upon your arrival, you might be asked to show proof of this onward ticket. You will also be asked how long you are staying (and if you have the onward ticket you know exactly how long that is). If you cannot satisfy these questions you will likely be denied entry.
Please note that these tickets don't necessarily need to be return/round-trip tickets - they can be onward tickets to another country altogether. They just need to satisfy the requirement to leave North America within the allowed timeframe.
