Here's a GIF for people who don't quite know what all 3 light entities do:
Keep in mind, light preview doesn't account for indirect lighting.
light_omni (point lights)
- Useful for quick, lazy lighting for your map when you want to test it
- For ceiling lights, wall lights, etc.
- Convincing and real-looking spot lighting, easily customisable
light_ortho (straight, orthographic lighting)
- Used for something like forced light creeping out of a window or anything
Hope this explained everything you needed to know!