It is not just a nice way of remembering the deceased by making him a part of you, it is also convenient, with dragons getting so big in their old age, burying them or cremating them wouldn't be practical, so processing and eating the body would avoid the problem of having a giant corpse rotting away.
Plus they also gain a lot of food and materials, it is even more efficient than burying the corpse so that the microorganism in the ground would decompose the body.