Because you can study not just the effects of the supplement, but also what kind of microbes are contributing to the types of odors. This study actually seems relatively decent I think.

Edit: Abstract from sci-hub copy.

The aim of this study was to evaluate the effects of proanthocyanidin-rich extracts from grape seeds on human fecal flora and fecal odor. Proanthocyanidin-rich extract containing 38.5% proanthocyanidin was administered to nine healthy adults at a dose of 0.5 g:day (0.19 g:day as proanthocyanidin) for 2 weeks, and proanthocyanidin-rich extract containing 89.3% proanthocyanidin was administered to eight elderly inpatients at a dose of 0.43 g:day (0.38 g:day as proanthocyanidin) for 2 weeks. Green tea extract and:or champignon extract, both of which have been found to have a deodorant effect on fecal odor, were administered in a similar manner as controls. In healthy adults, marked decreases in fecal odor and concentrations of methyl mercaptan and hydrogen sulfide in feces were observed during proanthocyanidin-rich extract intake, but the effects of green tea extract and champignon extract were weak. After 2 weeks of proanthocyanidin-rich extract intake, the number of Bifidobacterium had increased signifcantly (pB0.05), whereas the number of Enterobacteriaceae tended to decrease (p¾0.121). The level of putrefactive substances, including ammonia, phenol, p-cresol, 4-ethylphe- nol, indole, and skatols tended to decrease after proanthocyanidin-rich extract intake, and fecal pH also tended to decrease. Nurses and hospital aides performed organoleptic evaluations that showed less fecal odor in elderly inpatients with proanthocyanidin-rich extract intake than with champignon extract intake. In an in ×itro study, the proanthocyanidin-rich extract reduced methyl mercaptan and hydrogen sulfide release from the feces of healthy adults, and also reduced methyl mercaptan release from methyl mercaptan solution. The absorptive ability of methyl mercaptan was stronger in procyanidin oligomers larger than decamer than procyanidin dimer to tetramer. These results suggest that proanthocyanidin-rich extract from grape seed intake induces a reduction in the level of putrefactive products in the intestine, which may be linked to the modest change in the numbers of Bifidobacterium and Enterobacteriaceae. They also suggest that the strong deodorant effect of proanthocyanidins on fecal odor is due to the decrease of putrefactive products and the absorption of malodorous compounds from feces by the larger molecular procyanidin oligomers in the proanthocyanidins. Key words: proanthocyanidin, grape seed extract, polyphenol, fecal  ora, fecal odor.

Googled it and:

Gut microbiota: PAs can improve the diversity of gut microbes, and can increase the abundance of beneficial bacteria while decreasing the abundance of harmful bacteria.

Fecal odor: PAs can reduce the level of putrefactive substances in the feces, which can reduce fecal odor.

Fecal pH: PAs can decrease fecal pH.

Antimicrobial activity: PAs can have an antimicrobial effect on potentially pathogenic bacteria.

Prebiotic effect: PAs can have a prebiotic effect on beneficial bacteria.

Looks like I'm buying some seeded grapes for science!