This sounds a bit like the "Why pay for dropbox when rsync is free?" argument. Sure you can do all of that yourself, but for just $10 / mo (5->15) you don't have to worry about it. That's really what they're selling.

$120 / year for peace of mind that your production DB is backed up is well worth it for a lot of people, especially if the alternative is potentially a bug in a homegrown shell script which could silently fail catastrophically and lose your whole DB.