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For complex electronics, it was solder joints, sockets and etc get loose/corroded over time. Circuit boards and solder paste contains highly corrosive chemicals. By beating it, you are merely temporarily remaking the electrical connection until it "falls off" again.
Resisters are culprits too, they dry out over time. Every type of resister dries out and becomes crumbly inside, certain ones take a few years, some take decades but a quick punch can sometimes make the connection good enough to start working temporarily until ti heats up again, thus then needing another beating.
For CRT tube tvs, it was mostly the yokes and power supplies that needed the jolt from a beating.
For stereos, it was volume pots and mains voltage supply lines across circuit boards and of course, the IC's and/or transistor's solder joints loosening from heat and vibration.
I recently retired a PC that needed to be kicked sideways with a flat foot to get it to see the NIC card.. fun times.
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