It's just one more setting for the already extremely configurable Task Manager. For new users, it changes a default (the option to group is already there, with an extra toggle that activates it only when the task bar is full). I think the "cycle window on click" is a new option.
KDE is all about giving lots of options to users, but at the same time they aren't afraid to experiment and change its defaults to try to appeal to first-time users. In this case, window groups is a nice default that avoids scrolling or too-small buttons.
Before this KDE grouped them by default when task bar is full, it's one of the settings you have to change which take about 2 seconds. It's impossible to make a DE that fit all tastes with default settings, that's why we have settings
KDE is all about giving lots of options to users, but at the same time they aren't afraid to experiment and change its defaults to try to appeal to first-time users. In this case, window groups is a nice default that avoids scrolling or too-small buttons.