Quercetin is a flavonoid (a plant pigment) that is found in many plants and food such as onions, grapes, apples, berries, broccoli, citrus fruits, cherries, green tea, coffee, red wine and capers.
It is a member of the flavonoid family which is one of the prominent dietary antioxidants. There are many health benefits to antioxidants and consuming quercetin which include protection against various diseases and against anti-aging. 
People often choose to supplement with quercetin for many reasons, some include:
Studies have shown that supplementing with quercetin may help reduce inflammation, while a little inflammation can help your body heal from infections, chronic inflammation can lead to many illnesses and diseases. 
An 8 week study on 50 women with Rheumatoid Arthritis were allocated into a quercetin (500 mg/day) or placebo group for 8 weeks. Many symptoms were monitored and determined such as Plasma levels of high-sensitivity tumour necrosis factor, clinical symptoms including early morning stiffness, morning and after-activity pain, and tender and swollen joint counts were determined. At the completion of the study the results were that the 500mg per day of quercetin supplementation for 8 weeks resulted in significant improvements in the clinical symptoms and disease activity of patients with Rheumatoid Arthritis. 
Quercetin is known for its anti-inflammatory properties, its antioxidant activity, its anti-allergic properties characterised by stimulation of the immune system and its inhibition of histamine release. Test tube studies found that quercetin can be effective in late phase bronchial asthma responses, allergic rhinitis and restricted peanut-induced anaphylactic reactions. 
A search of the literature was conducted on exercise performance and quercetin. Eleven studies were identified where quercetin was tested on performance, exercise, endurance and aerobic capacity. The results were that there was a significant effect favouring quercetin over placebo. On average, quercetin provides a significant benefit in human endurance exercise capacity and endurance exercise performance but more studies should be conducted. 
A study was conducted on the impact of quercetin on blood pressure through a systematic review and meta-analysis of randomised controlled trials. The review of 9 human studies in 580 people found that those taking up to 500mg per day of quercetin and a reduction in blood pressure. These studies are promising but further studies are needed.