Why is it necessary to fire anything in an acting scene with a gun?

It’s not necessary at all, it’s just cheaper. The sounds and puffs of smoke (if they want) can be easily added in post production.

Movie gun violence is a big-money industry, valued by how it looks. There is a high market value on guns looking like real guns shooting, including recoil and ejecting cartridges and so on. It’s possible to fake some of that with special effects, but it’s also expensive work.

Just for the record here, a automatic rifle, like a M-16, cannot fire successive blanks, like the rifle is being fired in full automatic, unless the barrel is plugged.

I said the same thing. Sparks and sound can be added.

I just saw a play in which an actor pulls out a handgun and shoots himself several times as different characters. It scared the willies out of me after the Alec Baldwin event.

CGI costs $$$.

It would be nice if movies with violence and guns lost their appeal.

Good question.
Having said it, each person who handles the gun in circumstances like that is obligated to make sure it is safe.
Baldwin apparently did not and is in trouble in my opinion.