What's the actual purpose of terrorist attacks? I really don't get it, what's the reason for killing multiple people because of their religion?

Depends on the specific group. In the case of Islamic terrorism, the goal is largely to alienate muslim populations in western nations and make them easier to radicalize and make it more difficult for muslims to leave their native countries. The reason they want this is because for some time now, there's been a brain drain in the Islamic world. A large portion of their best and brightest leave their nations, and so with them goes wealth. So middle eastern nations fund terrorist groups who subversively make it harder for muslims to leave. They also fund terrorist groups as part of their wars against each other. Iran and Saudi Arabia have been fighting a proxy war for quite a while now. Sure the Koran is used as a tool to gaff up willing fools to fight these battles, same way Mohammed used it, and same way the Roman Catholic Church used the bible in the middle ages. Religion is one way to get desperate people to do things that seem unthinkably stupid.

The tactics they've been using have largely been successful too. That's why we're seeing more and more of it.