I am not saying that an insulation meter is not worth having, it is!
Just not useful for finding bad components imo.
Around here it used to be a law, a must have for all commercial repair shops, maybe EU has eased things up.
Like you said, to find a leakage and to be a final test before giving mains connectable stuff back to the customer.
Memory is telling that once I wondered if I should file spikes away from the edge of a desktop case, couldn't see them but they were constantly stinging.
Then for a forgotten reason I realised the case is live, can't remember how it was with the PE-wire.
It was a time before ATX and when mains went back and forth inside the case.
Luckily nowadays those fault current controllers are a must.