How Big a Hole??

[planter]

I know the rule of thumb for most trees and shrubs is to plant them at ground level or slightly higher and that works for me so I understand digging the hole the same depth as the root ball or pot and just loosening the subsoil.

My really question is this... Why do they say on a label to dig a hole the same depth but only 2 or 3 times as wide. I tend to make WIDE hole but not deep ones so whats the scoop. I alway incorporate a lot of native soil in but why does it matter all all how big around your hole is??? I just figure it gives the roots more places to go and most trees are pretty shallow rooted anyway.. Just observe trees blown over by the wind..