Analogies are an attempt to generalize an argument. Say we are trying to prove that "It will rain" is a valid conclusion from "If it is cloudy it will rain and it is cloudy". We can generalize this to "A -> B" & "B" -> "B", which you will recognize as an application of modus ponens from formal logic. Analogies try to use this power on fuzzier topics.
A normative principle like "We should build a new post office" are so abstract that they cannot be tackled head on (try defining "should" in the comments section and having no one point out a flaw to see why this is difficult). Because of this, we try to make comparison to other, seemingly similar, cases in an attempt to draw out the underlying logical structure. Because of this, I think they are a powerful and useful argumentative strategy.