Historical Theology studies the history of theology, especially the development of theological and religious ideas in their social, cultural, or other historical context. It's fascinating, important, and hugely influential. For Catholics HT was given a boost by the Second Vatican Council, which directed that seminary education in dogmatic theology should take the "genetic" approach:
Dogmatic theology should be so arranged that these biblical themes are proposed first of all. Next there should be opened up to the students what the Fathers of the Eastern and Western Church have contributed to the faithful transmission and development of the individual truths of revelation. The further history of dogma should also be presented, account being taken of its relation to the general history of the Church. Next, in order that they may illumine the mysteries of salvation as completely as possible, the students should learn to penetrate them more deeply with the help of speculation, under the guidance of St. Thomas, and to perceive their interconnections. They should be taught to recognize these same mysteries as present and working in liturgical actions and in the entire life of the Church. They should learn to seek the solutions to human problems under the light of revelation, to apply the eternal truths of revelation to the changeable conditions of human affairs and to communicate them in a way suited to men of our day. (Optatum totius, n.16)
These pedagogical instructions are an incentive, at least to me, to distinguish (a) systematics from dogmatics, (b) clerical education from theological education, and (c) overall formation from theological education. OT n. 16 is typically taken as a design for individual seminary courses: each dogma course begins with the Bible, proceeds through the Fathers and the twists of Church history, pays a visit to Aquinas, and runs into reflections on the life of the Church. I think we'd do better, however, to take this paragraph of OT as a plan for overall priestly formation. Ordinandi should soak in the Bible and the Fathers, should learn church history, and should be challenged (and already motivated) to connect the eternal truth with present-day questions. The speculative penetration of divine truth and the appreciation of the analogy of faith, under St Thomas's guidance, is the properly theological part of this education. This ought not to be restricted, I'd say, to a week or two or three in any given course: instead following Aquinas's pedagogy in sacred doctrine should be the main work of every theology course. I assume, however, that the other matters--Scripure, the Fathers, etc.--are being learned elsewhere, both in the classroom and outside it.
If systematic theology (i.e., theology) courses must all be taught genetically, then what about courses in Scripture, Patristics, History, Liturgy, Pastoral Theology, etc.? Will these have dogmatic and indeed Thomistic sections, since their material is, or contributes to, dogma? Hmm, I thought not. You could wreck a good theological education program by embracing OT n. 16 too vigorously.