A.K.A. Redshirts -- Action fodder.

If you're going to write action novels, there have to be casualties, you know?

So you have to have fall guys.  You can't kill off the main characters, usually, unless you're George R.R. Martin.  Or, if you do, it has to be a major plot point, usually toward the end of a book.

Meanwhile, if you're having battles and bullets are flying, well -- someone has to get it.

So you have to assemble a cast of characters that include a few Redshirts, here and there.

I find that I often have to go back and inserts scenes to introduce characters that I can later kill off...

Very cold blooded.  All writers are psycho's.