I think you're talking about using personas. I'm not surprised that was your reaction, as personas are often misunderstood, even by marketing folks.
The point of using personas is not to stereotype your customers but to establish focus sufficient to permit clear product decisions. In other words the idea is not "all our customers are like this" but "this is the type of customer we care about the most." Or more typically there are multiple personas, which allows you to create a matrix of product features with personas and again, establish priority for which feature to develop when, and how much to invest.
I'm using "product" and "feature" here broadly, so for example even in an ad campaign, each ad can be considered a "product" with "features" like where it appears, what's in the imagery, what the text says, etc.
You can run into trouble with personas if you start believing that they cover the entire range of potential customers. They don't, and again, that's the point: focus. That's why personas are sometimes given names and photos and detailed descriptions, to make it clear they're supposed to be individuals not stereotypes.