My guess is that someone saw the value (rightly) in being able to do "good" digital signatures on the web (better than docusign in terms of integrity/proof), and that meant (in their head) those certificates have to work in the web browser.

Which, if you don't understand web trust and PKI, means a bit of searching online will tell you that you need your browser to trust the CAs you use for digital signatures.

Which is of course not true - you can (and should) present an "untrusted" (i.e. not a server authentication) certificate as your client certificate or for signatures, as there's different trust bits and use-cases for different kinds of certificates.