The example code found on this section of the Cairo docs helps us understand why references created with the let keyword are fundamentally different to values created with the tempvar and local keywords and why references are sometimes revoked when performing jumps.
func foo(x):
let y = 1
jmp x_not_zero if x != 0
x_is_zero:
[ap] = y; ap++ # y == 1.
let y = 2
[ap] = y; ap++ # y == 2.
jmp done
x_not_zero:
[ap] = y; ap++ # y == 1.
let y = 3
[ap] = y; ap++ # y == 3.
done:
# Here, y is revoked, and cannot be accessed.
ret
end