In general, humans learn from experience.  Either directly, or indirectly through the experience of others.  So, we refer to previous examples of similar patterns and behaviours to explain new ones that occur.  The benefits of this are clear - it allows us to more readily understand and convey concepts without the need for a new frame of reference.  This is the use of metaphor.

However, there are also drawbacks to this.  Firstly, the word CONCEPTS was used above.  That's important as DETAILS aren't conveyed well with the use of metaphors.  If the details were the same, it wouldn't be a metaphor, it would be a recurrence of the exact same event.    As the details ARE different, that means there is a point (a level of detail) at which any analogy (metaphor) becomes redundant.

How do we know when the metaphor stops being applicable?
To deal with inconsistencies between the new situation and the metaphor used, tweaks are generally added to allow the continuing usage of the point of reference.  This is worth noting as at a certain level of detail, the effort involved in maintaining the metaphor will outweigh the value that using the metaphor at all adds.

Predicting the future based on the past
One of the most commonly used reasons for metaphor, besides giving a commonly understood point of reference is to predict what will happen going forwards based upon events that occured in the realm of the metaphor.  But these predictions may be affected and invalidated as the metaphor, by definition, has differences in the detail.  These differences can be of one of three types:

• The metaphor will have extra detail that isn't present in the current situation
• The metaphor will be missing detail that is present in the current situation
• External factors will have different weightings between the two situations (a discrepancy within a similar detail)

Why does this matter?  And how do these items link together?
By determining where the metaphor doesn't match the current situation, we can find the factors that will negatively impact the reliability of predictions, and take them into account when judging their merit.

The second thing to take away from this is that time should be spent judging at when level-of-detail any given metaphor is appropriate.  Top-down diagramming of a system and then finding the appropriate level up-front is a useful tool, as it allows you to see at what point you are going to be over-investing in the analogy, and where you are going to start drawing false conclusions.

Remember: metaphor and analogy are great at giving people a high level understanding of a new problem.  If they want more detail than that, then there is a price they have to pay in their grasp of the situation.