up-potting

Ok so multiple sources across all botanical practices deem up-potting to be an effective and common strategy. I am not denying that. However, I would like to better understand WHY that is. I have been searching for answers on this, yet to find anything really solid. The best answer I found was...

I currently have 25 plants vegging in 1 gallon fabric pots... regular non feminized seeds. My plan is to up pot them to 3 gallon fabric in a few weeks and flower in 5 gallon sub irrigated planters. My issue is I want to avoid; if I can, is moving all 25 to 5 gallon SIPs only to have to cull half...

I now have a few grows under my belt, and I'm learning from my mistakes and trying to improve. One area where I may be deficient is my up-potting technique. I currently start my seeds in 16oz plastic cups (with good success) and then transplant them directly into my 5 gallon airpots...