Originally a very early 1960's circuit, when transistors used to cost over a dollar each (then), smart designers were the ones who's circuits had less parts. This came from a time when oscillators normally used a unijunction transistor, so this was a much cheaper way of getting this 'UJT'.
Techy explanation: When the PNP transistor turns ON, it discharges the capacitor and it turns itself off, so the cycle repeats again.
Now the practical explanation which is much more fun!
The plumber was called in to fix a leak, he replaced the pipes and found 'some other work that needs doing' and did that too and his bank manager was happy that week. Later in the week, all the water was shut off in premises, and the central heating drained too, all he to do was wait until the last tank has drained down, then eliminate all the pipework in the heating system too. Still no joy, so now it must be a leak in the roof, maybe a collected pool of water somewhere, so the builder's called in to estimate the cost of replacing the suspended ceiling which has to be removed when the roof is redone. While measuring up, his mate, holding the tape measure, notices a wire and a battery inside a ventilation duct.
It was one of these bloody metronomes going blip every few seconds for the last 3 weeks!!!