Went to the S-Bahn station at Zoologischer Gardens yesterday and bought a one day travel card for €6ish.

Then caught the S7 service towards Alex Platz.

One thing I noticed was that there were no ticket barriers anywhere.
No-one checked tickets either.
Used various U and S-Bahn services throughout the day and nothing.
Same thing with the airport buses. You get on and the driver doesn't ask to see a ticket.

So, is there a point to buying tickets?