OK Nick may be I didn't expressed right.
You need the anchor for having something for the boat to hold on and for the cahin + rope to go as fast as poss in the bottom. But the chain is the important thing when you have rough or not so rough seas.
If you say got a danforth it digs in the sand or if you have a folding it catched on rocks but if you have a short chain the boat lifts the anchor out if is in the sand and if is rough it will brake the folding if if is stuck on the rocks. Therefore, you need a long chain to act as a shock absorber I suppose and avoid dragging or braking your anchor.
I b@#$ difficult to explain this in English or in any other language (you need to see it in the water to understand it) but I think you get the idea