It covers shared experiences decently well. Like obviously there is overlap but in the U.S. as a whole most millennials were kids who have a memory of a pre 9/11 world. Most zoomers grew up with WiFi being common in their houses. Most Gen Xers have memories of being a child near the end of the Cold War and were in the work force before Internet was common. Most boomers either served in the Vietnam war or have a memory of someone close to them going off to fight in an unpopular war.

A lot of those experiences have lasting effects in how those generations behave. It doesn’t mean everyone is the same but instead that you can follow trends that are more true for each generation

Then there is also the advantage of tracking a groups shared experiences like for example many millennials were relatively unaffected by the dotcom bubble but for the 2008 recession they were hit much harder