The Book of the Dead is a modern title for a collection the Egyptians themselves called the Book of Going Forth by Day, and the going forth in question belongs to the ba, the mobile aspect of the self depicted as a bird with a human face, usually the deceased's own. Confined to the tomb by night, reunited each evening with the mummified body it belongs to, the ba was believed capable of leaving by day, through the tomb's false door or its shaft, to visit the sunlight, receive offerings left at the tomb chapel, and move freely through places the person had known in life.
A substantial fraction of the Book of the Dead's spells exist to secure and protect this mobility: spells for taking the form of various birds and other creatures, for opening the tomb, for not being trapped or turned back, for having power over water and air. The anxiety these spells address is a real one within the system's own logic, a ba that could not leave, or that could leave but not safely return, faced a kind of second death, the very outcome the whole apparatus of Egyptian funerary religion existed to prevent.
The ba's freedom of movement did not mean the deceased ceased to depend on the tomb and its offerings; the ba required food as much as the ka did, and needed a functioning, identifiable body to return to each night, which is one of the central reasons Egyptian burial practice invested so heavily in preserving the corpse through mummification and providing statues as a backup vessel should the mummy itself be lost or destroyed.