The 2 D rings are designed to cope with the boat being towed, possibly at fairly high speed, maybe into the wind and sea. Subject to common sense limits, the D rings should stand up to anything - including anchoring in a heavy sea.
The plastic handle in the middle of the bow is designed to take light loads with no sudden shocks such as wheeling the boat around on shore.
When anchoring, a loop between the 2 D rings will halve the load on each and also tend to keep the boat facing the right way. Think of a standard bridle on a kite - a simple traditional kite usually has a triangular (2 legged) bridle between the line and the kite, for similar reasons.
If you anchor with plenty of line out, the line should hang down in a curve, partly under its own weight, and partly because of any current dragging on it. This curve in the line will then act as a shock absorber. Thus, the longer the anchor line, the softer the ride at anchor. Advice about how much line to use varies from about 4 times the depth of water to about 7 or 8 times the depth of water, depending on the weight of the boat, its windage, how rough the sea is and so on.
As for knots, when you look at them closely, they are often the same few moves in a slightly different context. For example, the common "round turn and 2 half hitches" is just passing the line twice around a post before tying a clove hitch around the line itself. An anchor bend is a round turn and two half hitches with the first part of the first half hitch going behind instead of in front. (You can't learn from this description, but I'm trying to say it's easy once you know how.) There are a few animated knot tying sites on the web.
Apart from anything else, tying knots is a satisfying skill.