Why not use the Internet?
Well the answer to that is tied up in the red tape DVLA have wrapped around SORN.
If you don't receive your VED reminder and can't find your log book (my paperwork system is haphazard at best) or have an old style log book for a vehicle that has been off the road since before SORN came into being (DVLA didn't issue new type log books for these vehicles) then you are forced to download and print form V890 and apply for SORN by POST.
As you can't interact directly with the DVLA data base via their website, even if you do apply online there is no guarantee that your SORN application will be processed. In fact take a look around the Internet and you will find a fair few examples of this.
It's very easy to assume that just because you are able to SORN your vehicle over the internet that everyone else can. Last figures I read stated that something like 10% of vehicles couldn't be SORN'd online.
Hope that helps answer a few questions.
James E Collins