What's the point of using shotgun slugs?

So I know that some places prohibit hunting with rifles since they can be used from so far away, therefore making a slug loaded shotgun a good choice, or perhaps someone wants to hunt larger game but they don't want to buy a rifle, but why do militiary and police units often use slugs in their shotguns? I thought the spread and stopping force found with buckshot were why shotuns are so useful, why not just use a rifle if you're going to fire a solid projectile?