These acids are called short chain fatty acids (SCFA), and they have been found to have MANY beneficial properties such as cancer protection and decreased risk for CVD.
What they do is decrease the PH of colonic contents, and increase normal cell proliferation which leads to decreased colon cancer incidence (that is why fiber is so beneficial).
The reabsorption of these SCFA’s contributes to only 2kcal/g and no, I wouldnt bother including them into your caloric intake because you can’t be sure just how much is actually fermented and absorbed etc.
Dont’ drive yourself crazy trying to count every calorie possible. The most important thing is to carfully follow the food guidelines you choose, and be consistant so you can accurately monitor the effects it has on your body.