The SCC invented a concept called Genuine People Personalities ("GPP") which imbue their products with intelligence and emotion. Thus not only do doors open and close, but they thank their users for using them, or sigh with the satisfaction of a job well done. Marvin is a prototype for the GPP feature, and his depression and "terrible pain in all the diodes down his left side" are due to unresolved bugs in his software.
Many galactic historians consider the invention of the fictional SCC to be amazingly prescient, seeing it as a metaphor for the all too earthly company Microsoft, which rose to prominence only some fifteen years later.