Generations are messy. Their dividing lines are blurry. Their connective tissue is lean. Their names have devolved into an alphabet soup of high-value Scrabble letters.

Everything untidy about generations brings witch doctors out of the woodwork with fanciful conjectures for enlightening the world about generational enigmas. There is no defending this voodoo. But nor is this reason enough to abandon the construct of generations, as some data purists are insisting.

There is more to generational analysis than the charade of pseudo-science that plagues it. In particular, generational tools can be invaluable for many consequential decisions, like in marketing.

Just to put it on the table, I’m co-author of two books on generations, co-developer of a generational database product and the ex-president of Yankelovich, the firm that pioneered generational analysis for business. So that’s where I’m coming from. It’s also where I’ve seen generations prove useful.

Hard-and-fast divides between generational groups seem arbitrary. Yet this is true of all social science. The jump from 20-24 to 25-29 years of age in a survey is no less arbitrary. It's true as well for race and gender. But categorization is how decision-makers make sense of social phenomena. All that matters is whether the categories work.

Some dividing lines are clear, though. Big one-year changes in both fertility rates and the number of births from 1945 to 1946 and again from 1964 to 1965 clearly demarcate baby boomers. This 19-year-long cohort invites skepticism that Bill Clinton and Michelle Obama could share anything in common. But demographically they do—the experience of numbers, and the associated power of sheer size in politics, business and entertainment. The surge of numbers was the first thing written about boomers in Newsweek, August 8, 1948. In 1965, the year after the baby boom, 36 percent of the U.S. was under 18; today, it’s 22 percent, an all-time low. No cohort has ever again enjoyed the boomer sense of itself as a singular force in society.

Dividing lines for other generations are blurrier, but there are meaningful differences in formative experiences tied to technology, the economy and diversity. Generations are about starting points—the aggregation of life trajectories that start together at a meaningfully similar point in time. It’s not where generations wind up but how they get there from their respective starting points. Not every person in a cohort is affected in the same way, but all are touched by the same, shared things.

Comparing starting points is the way to understand generations. For example, in 1967, The American Freshman Survey found 85.8% of incoming freshmen agreed “developing a meaningful philosophy of life” is an important college objective. It dropped below half in 1983. Over that same period, “being well off financially” grew from 41.9% to 68.7%. One could argue these are merely age-equivalent comparisons, but that’s just trying too hard to avoid calling them what they really are—attitudinal reflections of the generational differences in starting points between boomers and Gen Xers, the generation that followed them.

Messy or not, generations can be useful. Advertisers know reaching the right people always means reaching lots of the wrong people, too. Generational tools like ours that enable marketers to reach more of the right people pay off with an incremental lift in sales that our clients are happy to pocket, messiness notwithstanding. With every sort of decision, the best solution is strictly a matter of utility.

Generations are a cultural phenomenon. People identify with them, and generational jokes and memes are stock-in-trade for stand-up, stump speeches and social media. We can’t ban generations from data analysis or Pew Research Center reports without losing a lens on one important way in which people make sense of the world.

Gen Z, the coming generation, has a profound sense of itself as a cohort at a messy turning point in the world. They have lived through two decades of more change in social values than at any time since the 1960s. They will be the first to face the full impact of declining fertility rates and sensor-based technologies. Most importantly, they know climate will saddle them with a messy future. Gen Z knows that generations matter because everything matters more for the next generation.