what the heck is it that our Cabinet heads do? And why do politicians want to become Cabinet heads, since you never get to be President? What is the satisfaction that Kathleen Sibelius (say) gets out of leaving the Kansas governor's mansion to become head of the HHS? I presume it's rational, but it's a bit opaque from my perch outside the Beltway.
FLG has been pondering this. There's two ways of thinking about this. First, there's the good intentions motivations. For example, Sibelius believes she could do more good for more people by being a cabinet head than governor of a relatively small state. (Which if true, and FLG thinks perhaps likely, then it just goes to show the extent to which power has shifted from states to Washington when overseeing a small portion of the federal bureaucracy is perceived as more important than being governor.) Likewise, there's the "I have a patriotic duty to serve when the President asks."
Second, there are the selfish or self-oriented rationales. The most obvious rationale, at least in FLG's opinion, is that a politician was going to lose their next election or they hit a term limit. In that case, it might be better to spend a couple of years as a cabinet head while trying to figure out the next move. Maybe they are looking to get out of politics, and think that becoming a cabinet head is more like figurehead, which it certainly can be if the occupier would like it to be, and will take it relatively easy for two or three years before retiring. But that's not terribly compelling. Why not just retire? Perhaps governors from small states believe that being in Washington gives them more access to the inside the beltway crowd, so as to position them better for higher office. FLG doesn't think this makes sense though. Governors, in general, get plenty of access if they need it. At least FLG would imagine. Moreover, governors seem to have the inside lane when it comes to presidential runs anyway. Lastly, there's the become a cabinet head to fill-in some resume hole. For example, in preparation for a presidential run, a Senator might want to become a cabinet head to temper accusations that they've never run anything or a governor might want to try to strengthen some topical or subject matter weakness. Of the two, FLG thinks the senator wanting executive experience makes some sense. A senator who served on some relevant committee becoming cabinet head is entirely understandable. They have some subject matter expertise, hopefully, and are probably passionate about the issue. Taking a cabinet job just makes sense. The governor story is less plausible. Usually, as FLG mentioned just a sentence or two before, becoming cabinet head means some knowledge of the topic. One doesn't usually have the opportunity to overcome a major weakness in foreign affairs by become Secretary of State or Secretary of Defense. A governor might be able to weasel their way into becoming Secretary of the Interior or something, but that'll mean bupkis in a presidential run.
So, in conclusion, FLG thinks most self-interested reasons don't make sense. Some stupid self-interested people might try to leverage a cabinet position toward higher office, but it doesn't make much sense. The only reason FLG can see is if the person really cares about the relevant issue. Or maybe to buy some time for planning if the person's future political path is uncertain.