Tomatoes, a fruit (technically a berry) from the species Solanum lycopersicum, is simply loaded with beneficial antioxidants and bioactive compounds. Tomatoes are most well known for their antioxidant lycopene, which is fairly interesting because it has a significant different structure when compared to most of the polyphenol antioxidants you get from other foods. (Diversity tends to be good!) It's not alone though since lycopene as a supplement isn't as good as eating the whole tomato!
The main activity profile of lycopene includes antiatherosclerotic, antioxidant, anti-inflammatory, antihypertensive, antiplatelet, anti-apoptotic, and protective endothelial effects, the ability to improve the metabolic profile, and reduce arterial stiffness. (10.3389/fphar.2018.00521)
Both in vitro and in vivo studies have elucidated the potential of tomato against variety of metabolic syndromes. Latest research highlights the relationship between consuming tomato and its products with reduced risk of various maladies like obesity, hyperglycemic and hypercholesterolemic attributes, cardiovascular disorders, and cancer insurgences. (10.1080/10408398.2012.657809)
Tomato also contain other active compounds, namely, neoxanthin, lutein, α-cryptoxanthin, α-carotene, β-carotene, cyclolycopene, and β-carotene 5, 6-epoxide. (10.1080/10408398.2012.657809)
Lycopene is actually fairly poorly absorbed into the body in its native state. When you heat it, however, lycopene tends to isomerize, which makes it much more easily absorbed. Translation: If you cook your tomatoes, you can get more lycopene out of it and into your body!
The research is a bit inconclusive, either they haven't shown that lycopene reduces cardiovasular disease risk, or it does so by measuring indirect metrics like DNA oxidative damage or lipid peroxidation. (10.1017/S0029665116000744) Additionally, many of the clinical trials are underpowered. The basis for why tomatoes are good for you is based on the phytonutrients that aren't necessarily unique to tomatoes. There were even some studies that showed that lycopene alone wasn't very effective, and that the whole tomato was needed in order to show significant benefits. This sort of debunks the idea of taking lycopene supplements, but supports the idea of eating tomatoes regularly for health.