reboot during upgrade

sorry guys for the oftopic but can someone tell me why this guide from
/use/src/Makefile:

#  1.  `cd /usr/src'       (or to the directory containing your source tree).
#  2.  `make buildworld'
#  3.  `make buildkernel KERNCONF=YOUR_KERNEL_HERE'     (default is GENERIC).
#  4.  `make installkernel KERNCONF=YOUR_KERNEL_HERE'   (default is GENERIC).
#       [steps 3. & 4. can be combined by using the "kernel" target]
#  5.  `reboot'        (in single user mode: boot -s from the loader prompt).
#  6.  `mergemaster -p'
#  7.  `make installworld'
#  8.  `make delete-old'
#  9.  `mergemaster'
# 10.  `reboot'
# 11.  `make delete-old-libs' (in case no 3rd party program uses them anymore)


tells me to reboot the machine twice?
after 4th step and then after 9.

what's the point? is it dangerous to do it only once after 9 step and
not after 4?
what consequences would appear if i'll reboot the machine once during
the upgrade?