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.)