Omega-3 fats, especially EPA and DHA from oily fish such as salmon, mackerel, sardines and trout, support heart and brain health. General advice is to eat a couple of portions of fish a week, including at least one oily, which usually makes routine supplements unnecessary for healthy people.
Supplements can be reasonable for people who eat little or no fish, including some vegetarians and vegans (algae-based products provide DHA and EPA without fish). Evidence for high-dose fish oil reducing heart attacks in the general population is mixed, though specific prescription omega-3 products are used in some patients with very high triglycerides under medical care.
High doses can cause a fishy aftertaste, stomach upset and increased bleeding risk, which matters if you take blood thinners or are facing surgery. Check with a doctor or pharmacist before high-dose use, especially with other medicines.