From Science:
When my father was very young, he got a choice piece of career advice from his father: Pick something to do, and be the best in the world at it. My grandfather was a caterer who specialized in elaborate ice cream statues that were the highlight of the finest catered events in Baltimore, Maryland, from the late 1920s through the 1960s. Taking his advice, my father went on to become a leading volcanologist with a specialization in the eruptions and deposits of undersea volcanoes. My grandfather’s advice served my dad very well in his scientific career.
Our scientific community values and esteems expertise. Being the “world’s expert” on something holds a unique cachet, even if that something is extremely narrow. In graduate school, we are encouraged to plunge deep into a subject, to become the world’s expert, and, using that expertise, to advance the progress of science. A colleague once joked that obsessive-compulsive disorder is the hallmark of a good academic.
The drive to make young scientists “specialists” is motivated by an earnest and genuine concern for their success.
More here.