Why bake lights?

If you put a light in a scene, it works, so why do we need to bother with baking? There's two reasons.

First, each realtime light needs to draw everything it touches again. It means lights are really expensive to render, compared to a lightmap which is just stored as a texture.

Second, realtime lights don't have bounced light. Bounce lighting is all around us in the real world, with your average sunny day being lit just as much from the light bouncing off the Earth as from getting lit by the sun itself. Bounce lighting is the reason a window can make a whole room lit.

These two reasons make a really good argument for baking lighting. The only downside is that it'll make your maps larger, due to needing to store that baked data somewhere. Depending on what kind of map you're making, though, it's generally worth the cost. However, for larger maps, expect the lightmap to take 50-60mb of the filesize when optimised down.