Scientists distinguish between "splitters" and "lumpers," between those who favor fine-grained distinctions and those who tend to put entities together into broad categories.
For the most part, we are lumpers. Our minds have evolved to put things into categories and ignore or downplay what makes these things distinct.
Why does the mind work this way?
We lump the world into categories so that we can learn. When we encounter something new, it is not entirely new; we know what to expect of it and how to act toward it.
Someone without the right concepts might well starve to death surrounded by tomatoes, "because he or she has never seen those particular tomatoes before and so doesn't know what to do with them." Decartes' Baby. p.39-41.

Flexible thinking techniques allow us to be both, splitters and lumpers, depending on the problem at hand. They provide guidance on how to split and how to lump. For example, lumping iPhone with phones would cause us lose sight of its ability to run thousands of applications, including voice-based ones. On the other hand, splitting it from phones creates, at least initially, a problem for consumers, who might think that having a powerful mobile computer with thousands of potential applications on it would be an overkill for making simple phone calls.