It's just a different field of endeavor. One is immediately laboring to elucidate the meaning of Scripture as it stands in a particular place. The other is attempting to faithfully articulate the teaching of Scripture on a particular topic, on the basis of all its utterances. Each needs, and each contributes to, the other.
Yes, there have been several who have contributed substantially in both fields: e.g., Theodoret of Cyrrhus, John Calvin, John Davenant, Patrick Fairbairn, Charles Hodge. There are many others who have written in both fields, with varying degrees of success.