Yea but what if you had internal shorts inside a preamp tube or power amp tube or a tube rectifier and you were to turn the amp on , it wouldn't work and then the preamp section board at certain stages wouldn't work , so you have to use an oscilloscope to track it down
It's VERY, VERY rare to ever use a scope on valve gear, there's almost never any need - a basic 20KOhm/Volt analogue meter is all that's required.
Plus if the tube rectifier is has shorts or other problems it will cause the amp to blow fuses
A Tube Tester HELPS you out with this stuff to Cut down the troubleshooting
No, it's not something ever used in servicing - even if you have one you NEVER use it.
But why are you discussing valves?, they have been effectively obsolete for 50 years - you'd be better off learning basic electronics than confusing yourself with irrelevant antiques.