This shows the apps that are running and how much memory they are using, along with how much RAM on average an app is using.
Obviously the more memory used, the slower your phone will run, so identifying memory hogging apps and closing them will actually improve the lot of android users.
Lollipop suffered from memory leaks and apps which sucked up memory and drained batteries even when the devices were not being used. It does have memory management tools but these were of limited use, hard to find and people didn't.
Developer Preview 2 also adds in a few other changes. There is the Android M app launcher, which scrolls vertically rather than the more standard horizontal direction of previous versions of Android. It has lost the letters that allowed you to jump alphabetically to apps.
Themes has been hidden away. We are not sure what that would do, or how it would be useful to god or man.