I've seen it mentioned on the Paizo forums that the Graul homestead encounters would likely have be presented as optional, if they had though to do that in the first few Adventure Paths. The thinking apparently being that the Grauls are a sideline from the main adventure through-line of "rescue Fort Rannick, save the dam, defeat Barl Breakbones". Myself, I disagree with this, based on my play.
The Graul encounters were important to change the general tone of play and the shift in general deadliness that occurs once you start fighting giants. While there's elements of horror in the first two adventure installments, the shift to slasher-film and gore in the third is something that, I think, needs to be gradually introduced.
The descriptions and situation at Fort Rannick are pretty graphic and paint a picture of ogre society that is extreme. Without the transition provided by the Texas Chainsaw Massacre feel of the Graul, I suspect that the appearance of such grotesques would be very jarring.
Another important role the Grauls play is introducing the really high damage output of the giant-types. Up til now, the players have mostly been fighting human-scale opponents with human-scale damage. With the bump to giants, suddenly they face the possibility of one-shot kills (ogre hooks are, by the way, very nasty weapons). The ogre-kin of the Graul provide a little warning of what the party will be facing for the rest of this adventure, and most of the next.
The Grauls themselves are opportunistic hunters. Many folks wonder how they could surive so long, so near civilization. I suspect its because they're selective and careful about who they take, and that they probably have a fairly large hunting ground. What we're seeing in the Adventure Path is the Grauls having lucked into a find – a group of wounded and vulnerable Black Arrows. It isn't their normal modus operandi to grab a large group like that, but given the state of the Black Arrows, they couldn't pass up the opportunity.