NY presented the following explanation :
The instructions tell you to test it on a known-live circuit before using it
to determine whether a test circuit is live or to trace where a live cable
goes.

You can test one by rubbing the tip up and down your arm, to create
static.

But it needs to be very close to a live wire or pin, and doesn't detect
wires in a conduit in a wall.

Their range is very limited, which is useful to identify L from N in
bunches of cables, or in flexes. The range can be refined even more, by
placing your fingers around the tip as a screen.

When did building regs (or normal electrician
practice) change from using metal to using plastic conduits? I can imagine
metal conduits would block any volt stick, especially if the conduit was
earthed.

Why would you expect them to work through metal or screening? Either
metal or plastic conduits are in use.