The ushabti is a representation of a certain aspect of the
Egyptian concept of
soul, which was a lot more complicated than our modern one. It represents that part of the
deceased which went around the physical
drudgery of everyday life before
mummification.
Its function in the afterlife was to take upon itself any action or task that the mummy doesn't want to preform itself, so naturally manual labour comes into it. However the ushabti is also the mummy's personal servant and is responsible for keeping it comfortable and well taken care of. There is an amusing little text of a man having an argument with his lazy ushabti from the Middle Kingdom period, admonishing it for not wanting to go and fetch him beer, with the ushabti basically snuggling deeper into the covers and saying "what have you ever done for me, eh?"