Think 'basics' first...
- The sun (far away) is a virtual point source 'main', casting hard edged shadows
- The overcast sky is the virtual 'large source' 'main', casting few shadows or shadows with very soft edges.
- The light which simply bounces off nearby things (maybe light bouncing from the building across the street, or the light bouncing from the walls inside the room) acts as the 'fill' in the natural scene.
Now apply the 'basics' to the photographer's employment of artificial light (because the ambient light is simply not strong enought to take a picture)
- The 'main' source could be a small desk lamp or one of your speedlights or one of your studio heads, and all will be suitable equivalents to the sun in the cloudless sky.
- The 'main' source could be 'enlarged' to be a 'large source' by the addition of a diffusion panel or a softbox or by bouncing it off an umbrella or a big white panel.
- The 'fill' source can be a second active source or it can be a passive reflector that bounces back light from the 'main' source
Then let us think about 'how big?' a light modifier to use, either for a 'main' or for a 'fill'
- A 12" softbox at 12" from subject is just as soft as 24" softbox at 24" from subject is just as soft as 48" softbox at 48" from subject is just as soft as a 96" softbox at 96" from subject
- The key difference between the four 'large sources' is the light intensity FALLOFF due to distance. If something is 12" filxed distance behind the subject, it will appear to get much less light from the source at 12", than if it were illuminated by the source at 48" !!!
- A secondary relationship is the amount of area illuminated by a 12" softbox vs. a 48" softbox...BOTH have light which is cast LARGER than tha softbox's face, so the net benefit of 'larger' is somewhat diminished by that effect.
NOW pick a modifer, once you have fully understood ALL of the above.
Light modifiers merely allow you to achieve what you see in the everyday world, but to be able to CONTROLLABLY CHOOSE what you want to achieve in any particular circumstance, not depending upon the timing of nature.
Different modifiers CLAIM to have significant advantage over other somewhat equivalent product, but not all of them are strong reasons to choose that product...there is a lot of hype and empty marketing from many vendors.