How did smacking electron tube TVs help?

This practice is generally known as 'Percussive Maintenance'.

Any touching contacts, for instance in connectors, valves and their bases, and between the wiper of a potentiometer and the track, have a tendency to build an insulating film between the contacts. This happens most readily at higher temperatures, in high humidity, and when there is airborne contamination, especially sulphides. This can introduce higher resistance, non-linear behaviour, or break contact altogether. It can produce intermittent behaviour, changing with humidity, or voltage across the junction.

Sending a mechanical shock through the equipment can move the contacts with respect to each other, disrupting the film, and restoring contact.

In the case of a TV with a loudspeaker in it, sometimes the vibrations from the audio will change the contact state.

High contact pressures, and gold plating, improve the reliability of contacts against this sort of problem. TV valves, because they got hot, were especially vulnerable, and of course they aren't used these days.

It's worth noting that older TV's were constructed with point-to-point electronics soldered by hand, lacking a firm place for the components to be anchored to, such as this image showing the underside of the chassis of a 1948 Motorola VT-71 7" television.

(image taken from the wikipedia page for Point-to-point construction)

From the image alone it is clear why a solid whack to the box could provide enough energy to move loose components and bring them to rest in such a state they go back to working correctly. It could be down to exposed conductors touching, or a poor solder joint being loose and the movement provided from a solid hit moves the offending components back into a position that they work again - by either breaking connections that shouldn't be connected, or by doing the opposite to the components that should be connected.

Bad connections, corroded vacuum tube sockets making poor contact, cold solder joints and so on could sometimes be temporarily mitigated by the judicious application of 'percussive maintenance' techniques.