One shape, several paint operations
SVG gives an element one fill and one stroke. A two-tone outline, a sticker border, a keyline over a flat fill - all of them need more paint than that, and the usual workaround is to stack duplicate shapes and keep them in sync by hand.
The Appearance panel makes the paint a list instead. "Add Fill" and "Add Stroke" grow it, rows drag to reorder, and the eye on each row hides that pass without discarding its colour. A shape with one fill and one stroke keeps exactly the file it had before - the list only appears once there is something extra to say.
Every control edits the row you selected
Selecting a row shows that layer's own controls: its colour, its opacity, and for a stroke its width, joins, caps and dash pattern. Each layer can take a gradient or a pattern of its own, so a gradient sheen can sit over a flat fill without touching it.
Effects work the same way. Each effect in the Filters panel has an "Apply to" that names the paint layer it acts on, so a glow can belong to one bright stroke while a drop shadow belongs to the shape as composed.
Save a look, keep it linked
"Save as Style" in the Appearance panel stores the stack in the Library panel's Styles tab under a name. Applying it to another shape links that shape to it rather than copying - so editing the style later changes every shape wearing it at once, with no re-application.
What each shape keeps for itself is its own core fill and stroke. A style shares the structure of an appearance while a dozen icons wearing it keep a dozen different colours. "Detach" breaks the link and leaves the appearance exactly as it looks.
It exports as ordinary SVG
A stacked shape leaves the editor as its geometry defined once in a defs block and referenced once per paint pass with a use element, wrapped in a group. That is the SVG painter's model (SVG 2 section 3.3) doing what it always did - successive paint operations, each compositing onto what came before - so the file renders in any browser with no editor-specific markup.
Graphic styles are the one thing SVG cannot name: it performs a multi-pass appearance but has no construct for naming one and referencing it the way a linearGradient names a paint server. So a style is resolved when the file is written, and each shape exports the passes it paints. Reopen the .svgs and the link is still there.