Citric Acid is an intermediate in the Krebs Cycle, and technically crucial for cellular functioning and energy production. Its usage in supplements includes useful things like being bound to minerals (such as Calcium Citrate or Magnesium Citrate) to increase water solubility.
Citric Acid is most often used for
Overview and Significance in the body
Citric acid is an intermediate in the Citric Acid Cycle (TCA, or Kreb's Cycle) in the mitochondria and is synthesized in the first step via the combination of oxaloacetate and Acetyl-Coa via Citrate Synthase (to produce Citrate). It is the first step of the TCA cycle, but not the rate limiting step (which is the decarboxylation step of isocitrate dehydrogenase, or step 5).
Citric Acid and Acidity/Alkalinity
Citric acid is commonly supplemented as an aims to reduce body acidity, it has once been hypothesized that an acidic diet leeches alkaline minerals from bone to counteract changes in body acidity, thus reducing bone mass (as most minerals deposited in bone are alkaline in nature).
In regards to bone metabolism, one study found that low (0.72g) and high (2.16g) dose potassium citrate supplementation has not been shown to have a significant effect on reducing the effects of osteoporosis and the loss of bone turnover in and of itself although it seems to be able to ameliorate calcium losses induced by a high sodium intake. Another short-term study, using much higher doses (0.1g/kg bodyweight) found beneficial effects on bone metabolism as judged by urinary excretion.