Could it be to filter out the 50Hz mains interference?

It's an offshoot of the Mellotron[1] which was basically the original sampler. The mellotron was/is a keyboard where pressing each key spooled out and played back an individual tape strip of a real string section, cello, flute etc. It's the string sound that's really famous and recognisable. The mechanisms make a VHS look simple, see the wiki article for diagrams.

The linked machine is a read-write version of the mellotron, created especially for the BBC to do sound effects for radio programmes. One of the many one-of-a-kind cutting edge devices commissioned by the BBC in the glory days.

[1] https://en.wikipedia.org/wiki/Mellotron

You can build 'em out of essentially scrap parts fairly easily, actually. There's also dumps of the Melloton's sample reels floating around if you particularly want that sound rather than producing your own.

Peter Christopherson and Chris Carter of Throbbing Gristle made their own tape loop sampler out of car tape decks, some simple soldering and a keyboard controller in the late 70s. They could control the pitch, rewind, forward, etc. and otherwise trigger various tape loops which was the basis of the band's sound.

There's no reason to stop with tapes, though. MiniDisc is cheap, dead and has seamless shuffle (!!!) which means a whole stack of 'em could be chained to a mixer and played similarly. Still, you won't get the odd mechanical quirks and failings of using a cassette based system, especially if you start munging with manipulating playback speeds which may or may not be interesting to you.