why transplant???

ive been wondering this for a while and I'm sure there's a valid answer, why transplant? Ive got six plants grown from seed all planted in 3 gallon pots and everyone looks beautiful. Ive seen all these posts where people transplant like three times from seed to flower and the only thing I can think of is that you want the plant to get rootbound in a small pot so that when you transplant it has a dense root system, or for people with multiple grow rooms to save space. I might have just answered my own question, but I thought it was worth asking incase I was missing out on some beneficial process.

You started to answer your own question.
Losing light to the soil surface is undesirable, so having small pots and a continuous canopy is more efficient.
Letting the plant focus too much energy underground is inefficient- the roots SHOULD go out to the sides of the pot and just chill there for a while while the leafy part grows.
Unused soil zones within the pot can be wet and stagnant where the roots can't access the moisture in them. That's no good.
And when you are, as many many growers are, growing in a peat based medium, you want to MINIMIZE the amount of time that new roots are in old soil. When a product such as promix is packaged, it is buffered with lime to bring the pH up from the natural pH of peat (4.5-5.5 or so) into a more suitable range for common plants. That lime buffer is good for only so long after water has been added and it starts to react with the acids in the peat. After a couple months or so, many growers will note a SHARP drop in pH and sudden lockout symptoms. One way to avoid this is to constantly be potting up and refreshing the soil AROUND the root ball so that fresh roots can access fresh soil.
I hope that helps.

That makes sense, Ill remember that next grow