jtoots said:
I hear once runnings drop below 1.010 all you're taking off the grains is tannins and off flavors. So the sparge should stop there.
Last runnings is a bit controversial with some crossing over into religious fervor on the matter.
A brewer should really taste their last runnings to see. 1.008 is the actual accepted lower limit, but homebrewers tend to create ever upward safety zones, then begin to notice lower efficiency numbers. Some pro brewers (including me) can go as low as 1.004 with the right mineral content and mash pH, especially on lighter flavor and alcohol styles. It depends on the brewer, equipment and beer style.
OTOH, a few breweries have analyzed last runnings wort quality and found that below about 1.015 fermentability drops to almost zero, as does post fermentation flavor. Personally, I think it all adds up to the final beer, so I keep sparging.
Not all of my beers are sparged all the way out. Richer styles may stop as high as 1.024.
All of that is to point out that there is a huge range available to the brewer. So, you record your final runnings for repeatability, troubleshooting, stopping sparge and calculating efficiency.