Let's say you are watching an episode of Star Trek. Kirk, Spock, McCoy, and Billy Smith beam down to a planet to explore. Guess which one won't be beaming back up?
Sorry Billy, it's been real...
This is a useful trick; introducing a character just so that or she can die, creating a sense of real danger without having to worry about anyone actually caring that he or she is gone. We've all seen it, and I can even tell you how it goes, cuz it's the same in any house. You are watching a show or a movie, a character you have never seen before enters a very intense scene, and the first words out of your mouth are, "Well, he's going to die..."
Killing off your characters is fine, but you never want death to be a character's sole purpose, or they will feel like filler. If you do have a character who is destined to got the way of old yeller, try to give either them or their death a way to add to the story that goes deeper than just a short emotional reaction from readers or a quick way to add tension or stress.
Expending characters and having expendable characters are two different things. Avoid the later and your readers will feel far safer emotionally investing in your stories.