Except why have respect for the dead at all except for even remotely religious purposes? Without the concept of a possible hereafter, a dead person is dead, and that's it. Who cares what happens to the corpse?
Because it stinks. And it was your Grandma.
Respect isn't a religious concept, the last I checked.
Or, failing respect, there could still be love, and a wish to see a loved one preserved in one's last sight of her as near as to how she was when living.