Why a seed culture and a barm

If you're asking if you can incorporate your sourdough starter into your final dough, and make adjustments in the final dough because the starter hydration may be higher or lower than what the formula calls for,  sure you can.   Just be sure to hold back enough of your sourdough so you can refresh it.    The final result may vary from what the formula author intended, but absent it turning into a brick, it will still be pretty good bread.

Technically, the term "barm" refers to the scum/froth on top of fermenting malt liquor.  Peter Reinhart coined the word to describe a sourdough culture (and subsequently admitted in his blog that it is  a confusing and nonstandard usage).    It's a topic that's been discussed here before, and you'll find plenty of references using the search bar.

Here's a good explanation of the various types of preferments used in the baking industry:  http://www.kingarthurflour.com/professional/preferments.html

from one tablespoon to say a cup of starter (step) before combining it with rest of the dough formula when one could simply take that same tablespoon of starter and combine it with the whole dough formula?   Is that correct?

Or simply put... Why build the starter other than for flavour?

Try looking up elaborate in the site search box.  (My brain cells finally kicked in. I have a hard time remembering that word to use instead of "build." ) 

I can tell you that I keep a firmer starter than 100% and a rather small amount in the refrigerator.  I elaborate my starter so that I can bake with it.  In the elaboration, I correct for the hydration needed in the recipe.  Different hydration starters have different concentrations of yeast.  It is always good to refresh the starter (many times the first elaboration) before getting the starter working on a recipe.  One, you have some idea how strong the yeasts are and therefore a better grip on the various time adjustments your dough may take when comparing to the recipe.  Two, another reason is that the yeasts do their best when the bacteria they live with have a chance to acify the dough first.  By putting a small amount of starter in a whole batch of dough, it could be that the yeast will take a long time to build up their numbers and produce enough gas, meanwhile enzymes are working against you as your gluten matrix starts to deteriorate.  Yeast reproduce exponentially so that concentrating them in a small amount of dough first (even if the matrix is damaged)  and then adding fresh flour, the dough's integrity is preserved.  

From the moment water is added to flour, mother nature is trying to return it to basic components.  The idea is to raise and bake a loaf before it falls apart.  It's a race against time.  If all the dough was mixed with a small amount of starter, it may take the same amount of time to increase the yeast, but the dough may fall apart and get stringy before ever being baked.  Fermentation does take a toll on the dough eventually wearing it out.  Often a borderline over-proofed dough can be saved by adding a fair amount of fresh glutinous flour (and salt) and reshaping for a final rise before baking.  

Great explanation, Mini, I enjoyed reading it!  Another reason is that sometimes you want a higher or lower hydration level in your build to encourage specific microbes, such as protease in wet conditions or heterofermentive acid producers in drier conditions.  Building the starter separetly lets you select flours, temperature and hydration to encourage specific flavors, acids and enzymes.  Often the hydration choices and temps are quite different than what you would want for the final loaf.