Tieing roots in a knot, why?

[leatherback]

Besides the visual appeal (depending on taste), it'd also preserve the young plant's vigor cuz you just reduced the rootball without pruning more than half off

The whole point of reducing rootmass is to ensure maximum space for the roots to develop, and minimize the amount of old roots which contribute less to overall uptake than young roottips.

 

[RozendaalArts]

The whole point of reducing rootmass is to ensure maximum space for the roots to develop, and minimize the amount of old roots which contribute less to overall uptake than young roottips.

Of course but what's that got to do with my point? Old roots contribute less but still contribute and pruning them off would put the tree in recovery mode slowing growth. And by tying them up you still free up space in the pot for new roots. It's just a question of whether you want to sacrifice aesthetics for growth or vice versa, unless you like the aesthetic, then it's a win-win. You've already mentioned you don't, neither do I, but one can imagine that some might, especially commercial growers who prioritize a quick turn-around

 

[leatherback]

Of course but what's that got to do with my point? Old roots contribute less but still contribute and pruning them off would put the tree in recovery mode slowing growth.

I think you overestimate the effect of root pruning on growth, and underestimate how poor old roots are in uptake. The white roottips are pretty much responsible for most of the uptake. Everything else is anchoring and space
Besides that, trees where you want maximum growth should not be in small containers.