To help carry along the plot of the game until Tifa, Barret, and Cloud were developed enough to take control of the game.
I always thought that was what their purpose was.
And once the main characters were developed enough to take over the game they were not needed anymore. And that is when they were killed off.
I always viewed them as kind of like while the house was being built they were the support for the house. And once the walls were strong enough to take control for themselves. The supporting boards are not needed, so they are taken down.