In article 1, he defines circumstances as "conditions [that] are outside the substance of an act, and yet in some way touch the human act." (This means that the circumstances of an act are its accidents -- an accident being something that belongs to something but doesn't belong to the thing's substance -- but the term "accident" probably doesn't add much nowadays to that definition.)
In particular, circumstances touch the human act by being in the actor together with the act. So, as we'll find out, when an act occurs is always a circumstance of an act, even if when it occurs plays no part in either the actor's will or in the overall morality. (A thief who steals whatever he can whenever he can, for example, takes no account of the circumstances of when he steals, nor does when he steals something affect the morality of his theft.)
But I'm getting ahead of myself, since it's not until article 2 that St. Thomas proposes that circumstances are a matter of interest to the theologian. He gives three reasons:
Circumstances make human acts better or worse from a moral perspective (his proof coming later on), and this is a theological subject.
Ignorance of circumstances can affect whether an act is voluntary or involuntary (or non-voluntary), as was mentioned in article 8 of the previous question. An act has to be voluntary for it to be meritorious, and whether an act is meritorious is a question for the theologian.
I find it a little odd that he phrases the article around the question of whether circumstances should be considered by theologians, rather than considered by the science of theology. They're equivalent, as far as I can tell, but it seems more personal to talk in terms of persons rather than fields of study, and the Summa Theologiae is for the most part not a very personal textbook. My ignorant guess, based only on the repeated references to oratory in this question (especially objection 3 of article 2), is that the students he wrote for, having already studied circumstances in their rhetoric classes, really would ask the question that way: "Why is a theologian, of all people, talking about circumstances?"