At a high level, layers allows you to define a set of graphs where node and edge positioning stays constant but individual nodes and edges may not be visible in any graph within the set. (whew, the Graphviz documentation explains this better)
While Graphviz’s SVG output supports layers as best it can, SVG only shows you the “first” graph/layer (think of it as the graph on the top page of a stack of pages).
Does anybody care?
Is there a better user interface
What features are missing?