A regular digital document can be called into question in court with some concern for digital manipulation. Chain of custody, cryptographic signing, audit logs, etc help establish vendors as trusted in legal disputes. It’s nothing particular to them, it’s just compliance with UETA and ESign laws as well as state/local standards in the US.

As much as vendors like DocuSign provide UX and infrastructure for document management, they also package an aspect of legal compliance as a service.