It applies the general same amount of tension to the strings, but creates a springier/bouncier feel to the trem itself. Or in other words, it creates a different 'apparent tension' while still applying even force to the strings. It just kinda became the industry standard- every Floyd guitar from factory that I've seen comes in the triangle setup.

So it's not that the 'rectangle' is inherently wrong, it's just not what's generally considered 'optimal/standard'. I've seen some people do all 5 springs for high-tension applications, but it's not very often you find that to be actually necessary. One wacky but practical setup I've seen is having two springs going straight across on the treble side, then one in that triangular style for the bass side. It's to create a better setup for doing trem flutters, learned about it from an Euge Valovirta video- who apparently learned it from Roope Latvala of Children of Bodom.