So, there must be a reason.
On their site, they say they can "time stamps to the nearest second". They could be using their own blockchain to do that, but don't need to. They only need to send 1 transaction to the bitcoin blockchain containing a hash of their timestamp/file for every authentication they wish to make. I read that users can "see the Blockchain Key independently from our site at Blocktrail.com", so the verification is actually on the bitcoin blockchain. It make sense because it is the most secure blockchain available.
So why should I be using their service and pay also???