What is the point of a dual SIM phone?

So, I'm curious, what's the real deal behind a dual SIM phone? I mean, what's the point of having two SIM cards in one device? Is it just for convenience or is there more to it? Like, do people really need two different phone numbers for different reasons? And if so, what kind of situations would warrant such a need? Is it more of a business thing, or is it something that casual users could also benefit from? I'm just trying to wrap my head around the practicality of it all.