I have shot probably hundreds of thousands of rounds of pistol ammo, and reloaded a lot of it too. But if you only shoot pistol ammo, you do not really need to be concerned about the brass stretching and getting too long. But with bottle-necked rifle ammo, this is a different story.
When bottle-necked rifle rounds are fired, the brass tends to stretch and the brass gets a little longer each time it is shot and resized and reloaded. Eventually, the brass could get so long that the neck of the brass will reach all the way forward until it can reach the beginning of the rifling. If it does this, it can "pinch" the bullet in the case and cause very high pressures and even case or primer failure.
Therefore, bottle-necked rifle brass must be trimmed to a safe length.