First, self sacrifice does not have to be selfless or altruistic. What about the lawyers who sacrifice their time getting educated so they can have it all? Next, for me, altruistic self sacrifice is necessary for personal fulfillment, but others may feel fulfilled by working and providing for their family.
It shows that you are selfless and altruistic. If you sacrifice yourself to good than it is a sacrifice that can help the world to light a fire in their hearts. Also if you are denying yourself something for a greater good, possibly a longer lasting good, that's great and in fact a wise investment
Ideas, beliefs, and personal identity means nothing once you're dead. There is very little worth sacrificing yourself for, save perhaps the life of a loved one.
But for a political movement or a belief? You'd be more valuable to that cause alive than you would be if you were dead.
Self-sacrifice being necessary means that you have to sacrifice yourself for the greater good. The problem is that you can't force everyone to want to sacrifice themselves so some people, who may or may not be very cruel, might just sit around being very selfish. If it was necessary then these people wouldn't exist, but they do!