What paint should I use to paint front door? Why using Urethane in paint?
The world of paint is intentionally made confusing by people trying to sell you stuff.   (a lot like the world of investing.) 

One of their favorite tricks is taking buzzwords from better paints, and finding the minimum possible way to legally use those words in their product.   Like spar varnish - it's for spars.  Or Epoxy, which is 2-part by definition - as is Polyurethane -- Imron, Nason, Interlux and Awlgrip are examples of bona-fide 2-part polyurethane paints. Your car and any commercial jet is painted with it.  They work because the cure happens in-situ, at full strength and without the need for air or moisture to cure. 

Any 1-part paint is simply already-cured epoxy or urethane, ground up and mixed into the paint.  The material is tough and does help - but not near as much as the real deal. 

They won't sell you actual 2-part urethanes or epoxies because the B-part is a toxic immune system irritant until cured.  It requires careful mixing and extreme "moon suit" grade protection when spraying. (brushing is safe.) They will sell you 2-part garage floor paint because nobody sprays that.