...when installing you are bolting to a live buss unless you turn the main off. Had a bad experience installing a breaker in a bar many years ago I didn't shut the main off and my screwdriver slipped off the screw and faulted from the buss to the back of the panel....
For that reason, those are quite dangerous to work on.
I have melted a screwdriver or 2 from that same thing on those old Bulldog panels.
A lot of them out there are "split-bus" type, and you can't easily shut off the main to change out an errant breaker. Gotta work `em hot, or it's an all day affair waiting for the POCO to come and shut off the juice.
On another job, I did shut off the main first when changing out a bad double-30 breaker. This was at 4 PM. That main would NOT reset back on.

I had to do an emergency panel changeout that evening, since no replacement main breaker was available. This was over 20 years ago! They were obsolete even back then.
I put in a nice Square D type QO panel, but had to work until 10 PM that night to get them back on. The HO was not happy that a non-working dryer service call ended up having to replace the whole service panel.
FWIW, I can still get replacement Bulldog breakers from a local supplier. Just bought 2 double-pole units for over $60 each. Still made by ITE, but they are playing the FPE game: Jacking up the prices to encourage you to buy a whole new service, instead of replacing a few old bad breakers.