what purpose does that silvery stuff near the base of high pressure sodium lamps serve?

I believe it's there to remove impurities from within the outer glass envelope that would otherwise attack the arctube.

is there a reason why this feature is used only on high pressure sodium lamps?

how come its not needed on mercury vapor or metal halide lamps?

I've wondered the same thing.  Except I have no idea either!  And on the crappy "Polarlites" Chinese HPS lamps I have, the silvery stuff only goes about 3/4 the way around the bottom of the bulb.

May have something to do with the material HPS arc tubes are made of. The only reason I can think of.

so is getter required in ceramic metal halide lamps too?

Yes, but you see the getter as a disk or little puck attached to the frame.

what excatly is the getter made out of?

The pellet is Zirconium-Iron.
The silvered getter is barium.

thanks!!

why the new generation of lamps made this days don't have anymore that barium coating at the bottom?

The "flash" getters were used in vacuum tubes as well-Cesium.These absorb any further gas in the tube when it has been pumpted to as much vacuum as the pumping equipment can get to.The flash getters absorb gas the pumping misses.And the flash getters will still absorb gas ginen off by the tube or bulb elements as it is used.Bulbs or tubes used a lot show less "silver" color in them-they are still good-just that the getter has absorbed gas during the life of the tube.The getters are more used if the vacuum tube is used as a power amplifier-or of course in the HID bulbs.Mercury,MH lamps are usually filled with argon or nitrogen-not pumpted to a full vacuum.Those have the disc like getters.

I think your HPS lamp use solid state getter, similar to Philips SOX-PSG low pressure sodium, instead of the barium getter.
These getters are exists in all sodium and MH lamps. Only the American style probe-start MH lamps don't have them, since they are built like mercury lamps, so they requires low pressure nitrogen atmosphere in the outer.
The getter absorbs oxygen from the outer, to maintain the vacuum inside it. Otherwise, thermal insulation inside the lamp will be lower and the arctube seals would oxidize and fail very rapidly.