That's what caused me to get stopped by the police my first day in Berlin a few years ago. I wanted to go to a board game store, so I looked one up on Google in the hotel lobby, walked to a u-bahn station, paid a couple euros for a ticket, walked downstairs and got on a train.

And promptly got pulled off the train at the next stop for not paying.

They were very understanding when I explained I had only been in the country about two hours (and showed my passport stamp as proof). They explained that there are machines to punch holes in the tickets... I did think it was strange that there weren't any turnstiles, but I figured, maybe you run into them on the way out or something (and I had only been in a foreign country two hours, everything was strange).

FWIW the police does not perform ticket checks in Berlin. Traditionally it was done by employees of the public transport company and now increasingly by third party contractors.

It might have been some security guard thing, I don't remember. It was a few years ago.

Some stations in London can be so busy, that they'd need some way to prevent people from overcrowding the platform even if there were no barriers.

It's pretty common for smaller stations near major football stadia to have the barriers controlled manually after big games -- totally open for some minutes, then completely shut until the platform has cleared.

(London used to have a proof-of-payment system on the articulated buses, since you were allowed to board at any door, and it still exists on the trams.)