I'm confusing myself and everyone around me!

Sure; go for it.....

to do one of my long posts that "over answers" the question.

Yes, if you have somewhat more othan 11.8oz of "mother starter" (or storage starter or any one of our various names for the starter that we constantly maintain - I will use "starter") and it is properly mature - peaked, but not over ripe - and it has the same hydration as the "levain build" in a formula, you can use it in place of the levain build.  Just make sure that have enough to hold back so that all of the starter line does not get baked into the bread.

So why does Mr Hamelman write a formula like that?

Several reasons.

Remember he is writing for professional bakers who might not routinely maintain enough starter to support a large bake.  For them it makes sense to maintain a somewhat smaller starter and then build it up prior to an actual bake.

A "levain build" allows us to control the timing of its ripeness a little better.  Salt can be added to the build so that it matures a bit more slowly.  Different innoculation rates (that being the amount of seed that is put in the levain build) will cause the pre ferment to ripen at different times.  We would not want to put salt in our mother starter and frankly, for some of us, if we are just feeding our storage starter, we eyeball how much starter is fed, so a levain build gives us more control when we need it. Using the levain build at the proper stage of ripeness cannot be overemphasized.

The method of using recently fed starter directly in the bake works if the hydration of the preferment is equal to the hydration of the starter.  But what if you were baking three different breads - each with a different hydration on the preferment?  The easiest way to do this is not to maintain three different starters, but to maintain one and create the various hydration pre ferments via builds.

Levain builds allow us to maintain a starter at the hydration that we "like" - for ease of maintenance - yeast vs. sourness properties, etc - and then create a different flavor profile and baking qualities in the bread by creating pre ferments at different hydrations.

The last reason I can remember (I am sure there are more) is taking a small amount of seed from the starter and creating a build is that using that method it is extremely difficult to accidentally bake our entire starter.  For someone with a precious, very old starter, losing the starter is a nightmare and some of us maintain two or three backups so it is never lost.  This is just one more method to make sure it doesn't disappear. 

But that's why the formulas are written as they are - not to confuse or to make you lose sleep.

Hope this helps more than confuses.

Happy Baking!