It's also possible to engage the parking brake and release the footbrake - thus not dazzling those behind !

A number of cars go into neutral automatically if stationary with footbrake applied and re-engage the gears when the footbrake released, eg Vauxhalls.

It does save a small amount of fuel.

Not much in it with a slush box. Less mechanical wear if left in D; the slip will put some heat into the transmission fluid but there will be some theoretical wear on the actuation mechaniam and brake bands if you go into neutral and back. Neither should be a problem with typical usage.

With DSG you must keep your foot on the brake if you leave it in D, to back off the clutch. Handbrake does not disengage clutch (at least on ours) although I am told that the gearbox will disengage it as a self-protection measure after a period, but I haven't tried it.