The “why” of fish oil consumption can be either super complex or very simple. The complex side has been covered by books like the Omega Rx Zone and innumerable web sites that are only a google search away. The easy answer is fish oil is vital to life and makes everything you want to have happen, happen and prevents many undesirable occurrences. Sorry if that is dodging the question but an exhaustive review of fish oil is beyond the scope of the Q & A section.
Regarding varieties of fish oil Costco’s Kirkland brand is good as are Jarrow and Nordic Naturals. A good daily dose ranges from 3g of EPA/DHA if you are pretty healthy to 10g per day if you have some health problems. If you use blood thinners tell your doctor you want to start using fish oil.