What does the liver actually do?

Several hundred distinct jobs, which is why the question is hard to answer briefly. It makes bile for digesting fat; it manufactures most of the proteins in blood plasma, including nearly all the clotting factors; it stores and releases glucose to hold blood sugar steady; it stores iron and vitamins; it breaks down old red blood cells; and it processes essentially everything absorbed from the gut before the rest of the body sees it.

That last point is the design principle. Blood from the intestines does not go straight into general circulation - it goes to the liver first, through the portal vein, so anything absorbed is inspected and altered before it reaches the heart and brain. It is the reason many drugs have to be injected rather than swallowed: taken by mouth, the liver would destroy them on the first pass.

It is also the only internal organ that regenerates substantially. Remove up to two thirds and the remainder grows back to full mass within weeks, which is what makes living-donor transplants possible. What it does not do is warn you: the liver has no pain receptors in its tissue and can lose most of its function before producing symptoms, which is why cirrhosis is so often found late.