To get new loot. Not get the same loot over and over.
If I simply wanted to watch numbers go up in meaningless ways I could start a stopwatch or install some idle clicker game. The thing that should change with new loot is gameplay, not merely numbers+1.
They can already do that by simply allowing all those legendaries to drop while lvling, and not guaranteeing the players will get them always, since then there is nothing to farm for in endgame.
Hopefully there are not any significant amount of better and worse legendary effects on purpose? Why would players want to use the intentionally worse items if given the choice? Shouldnât the goal (which might of course not match reality, as balancing can be hard) be that all legendary effects have the same power level, for maximum player choice and build diversity 