Keep it simple, but not too simple!

I read some articles about Extreme Programming.  (It’s a kind of process in software development). When the author talk about how many documents we need in software development.  He said: Keep it simple, but not too simple!

It’s a very interesting sentence.  Some people don’t make sense.  I comprehend it in this way. 
For example: 
A team wants to construct a building.  They get enough bricks and other stuffs.  All the things are enough for they to construct a building.  But they are afraid the bricks are broken, so they buy lots foam to protect the bricks.  Obviously, they don’t need the foam.

May be you would say: it is a stupid story. But in our real life, we always do stupid things.  We always add some unnecessary things.  In this story, we don't buy and use foam that let us keep it simple.  In the other hand, we must buy bricks, it is necessary for that team to construct a building.  So don’t keep it too simple!

Pay attention to the important things in our lives, ignore the unnecessary things.  That’s my opinion.