The last lesson in this chapter, Lighting a Bedroom with Eevee, will help you understand the use of multiple irradiance cubes. Using them to illuminate areas of the scene.
Because of the difference of how light behaviour, in particular bounce light, is calculated between Cycles and Eevee, we can use irradiance cubes to help create more realistic lighting with Eevee.
Think of each probe as taking a reading of available light at each probe location. By adding one big cube, as Spikey said, you will have probes inside objects. This can effect the light, and shadow, around the object. Depending on how close the probes are, and you definitely don’t want probes intersecting the surface of objects.
