That's probably part of it, but it's not the whole story. Most people know that their political views are very unlikely to have significant effects on the behavior of the government, much less its reach into their own lives (yes, I know, here I am blathering away as if it were otherwise). So here's another angle: emotional capital invested in political (/religious) beliefs is safe.
My mistaken beliefs about everyday reality have serious practical consequences; they are their own punishment, so it is in my interest to correct them. I had better get the behavior of unsupported objects right, or I risk dropping a hammer on my toe. Reality doesn't spank me, however, for my mistaken beliefs about history, political economy, or the origin of the universe. I will not endanger my toe if I mistakenly believed in the efficiency of markets or the historical inevitability of communism or the second coming of Christ. The predictable--and, in a sense, rational--result is that I tend to choose my religious and political beliefs for reasons other than their predictive/explanatory power (that is, their truth). The ideas I latch on to might just be in the air, evolving analogously to genetic drift. More interestingly, these ideas may do a lot for their hosts, and be subject to a lot of selective pressures. They may comfort, flatter, and justify me, and give purpose to my life. They may project to others my benevolence, cooperativeness, or respect for authority. They may strengthen my bonds with fellow believers and focus our collective hatred. They may reduce cognitive dissonance with the hogswallop I must recite to avoid persecution.
This kind of non-explanatory "belief" formation might be rational, in the utility-maximizing sense, but it is certainly not honest. Entertaining these notions as merely beautiful or inspiring or conventional in my tribe doesn't get the job done; I must also, in some sense, "believe" them--that is, take them to be true. And the dishonesties compound; a cherished "belief" must be protected from arguments and evidence that undermine it, whether by averting ones eyes or by gouging out those of heretics.
I keep putting "belief" in quotes because I don't think
- my belief that I am sitting at my computer and typing and
- my "belief" that I will be fed peeled grapes by seventy-three virgins (one better than that other religion down the street!) in the afterlife
- If asked, I say that I am sitting in front of a computer.
- I behave as if I am sitting in front of a computer. I wiggle my fingers over the keyboard in a way that would be strange and useless if I were not. My certainty that I am sitting in front of a computer could be calibrated by my willingness to bet on it.
Other notable properties of "beliefs" include a sense of commitment, of personal importance to the holder, and unwillingness to be swayed by (or even active avoidance of) evidence or argument against. If someone challenged my belief (1), I would be surprised, but probably not angry. If I found out that this was not a computer, but, rather, an empty box wired to some distant computer, I would be very surprised, but my identity would not be threatened. Beliefs can be held, but only "beliefs" can be cherished, or threatened, or cause incompatible beliefs to be treated as heresy.