GA_DrawInfo : For which classes ? Is it worth ? Does it makes things better ?

Online Status

Hello, at Gadget level we see:

#define GA_DrawInfo (GA_Dummy+33)
/* (struct DrawInfo *) Some gadgets need a DrawInfo at creation time */

... "Some Gadgets" : Which exactly ? I've seen that used for Buttons, but i'm not sure if it is mandatory or not.
Also what other gadgets ? Anything that draws ?