This sounds quite helpful for people who want to upgrade from P to Q or Q to R - if both source and target version support seedvault well enough that at least these manual steps work 
When I tried seedvault with LOS it also worked perfectly fine with an USB OTG adapter and a normal USB flash drive. I find it quite reassuring to have the backup on a different physical device in case of loss/damage to the phone (I also had lots of SD card failures over a handful of years so my trust in them is limited 
).