I would like to understand the difference (and underlying reasons for any difference) between pitching a single vial of yeast into 5.5 gallons of wort vs. taking a half-gallon of that wort and making up a starter, then pitching the yeast into the remaining wort. I do not presently understand. I'm a physicist and would like to be further educated in matters of brewery.
From what I have read, one might view the process as fencing off 10% of the brew volume, pitching the yeast vial into it, and then at some nebulous time later (at or after high kraeusen) removing the fence. Somehow the confinement of the yeast (the only difference I can see) results in something different from simply pitching the yeast vial into the original wort volume.
My most recent batch (#8 after a 30 yr hiatus in brewing) was also my first >5 gallon batch. I made up 11.5 gallons of 1.044 wort for a saison-like brew. I diverted 1.5 qts into a half-gallon glass jar, pitched the very fresh vial of WLP585 saison iii blend into it, and watched it while I gathered the rest of the wort into a 15 gallon fermentation vessel. Very few hours later the foam on the starter had risen to about 3", very nearly overflowing the starter vessel, so I pitched the whole thing into the 11 gallons of wort. Within a few hours, vigorous fermentation had begun (which means to me that the wort had become saturated with CO2 so that the excess was bubbling out of it). By the morning of the second day (75F ambient temps, no chilling of the wort prior to pitch -- it was just room temp) all signs of fermentation had ceased. The sp. gr. measured 1.006, an apparent 86% attenuation in 40-ish hours. In the few days since then, no further attenuation has been apparent, and the beer is holding 1.006. This mash did not exceed 150F, so I expected to have a high fermentable fraction of sugars, but that appears to be a phenomenal fermentation rate for a single vial of yeast in 11.5 gallons of wort.
The use of a starter-like process with so short a starter growth period seems to me to be indistinguishable from simply pitching single vial of yeast into the original wort. Now, that's likely enough part of my lack of understanding. I could perhaps understand if someone let a starter develop to full attenuation, let it floc' out, and decanted the liquid prior to pitching the starter into the wort. But why discard the (in principle overpitched) starter beer? The only function I can guess at would be to carry off the yeast reproduction products. I could perhaps understand if there were some semi-magical difference in yeast characteristics from growing up in a limited environment and running out of food prior to floc'ing out. Maybe. But to my very linear mind, I don't understand the workings of making up starters to attain a certain pitch rate. Do understand, I say "I don't understand," and am not saying there is no difference. It's a good day when I learn something, and I'm hoping this august group can "make my day."