Fat won't lower blood glucose because if you eat, say 23 grams of carbs alone or 23 grams of carbs and a tablespoon of oil, you still consume the same amount of glucose regardless, that will eventually get processed. It should be slowing the rate of absorption a bit however, which lowers the IMMEDIATE insulin response somewhat.
It's been said that this can be useful because it eliminates a large, sudden spike which can contribute to insulin resistance if it happens often. This is also why we are encouraged to consume more "whole grains" which have fiber that serves a similar purpose, for whatever that's worth.
This is also useful because a lack of such a sudden and large spike in blood sugar could potentially contribute to a more stable blood sugar throughout the day.
But here: European Journal of Clinical Nutrition - Abstract of article: The effects of fiber enrichment of pasta and fat content on gastric emptying, GLP-1, glucose, and insulin responses to a meal
"Conclusions: A dose of 1.7*g psyllium did not evoke measurable effects on gastric emptying, postprandial GLP-1, insulin or glucose metabolism. However the addition of 30*g of oil and 3*g of sodium propionate to the pasta did reduce gastric emptying, increase GLP-1 and reduce glucose and insulin concentrations. While this short-term study may have implications in terms of reducing the risk of diabetes and improving coronary risk factor profiles the long term effects of these nutrients need to be studied."
Also: Cambridge Journals Online - Abstract
"Addition of fat to either component of the meal reduced postprandial blood glucose (P < 0.05) and insulin responses, but when the fat was incorporated in the soup, peak glucose and insulin responses were delayed as well (P < 0.05)."
So... Fat slows and delays the absorption of carbs if these studies are to be believed. It also has the effect of lowering the overall insulin response. Whether this is good, bad, or neutral for the relationship to insulin resistance is not something I can answer, but while I was eating CW-style, I was constantly told that a giant, immediate spike in blood sugar was bad. Of course, primal eating has made me begin to question this.