The motivation for this option is that there are some features missing in Discourse when SSO is enabled, most notably to invite someone by email (not only by username) at the end of the topic. In this scenario, when you enter an email to the invite field, Discourse will send an email to the invited person with a link to the external registration system (not to itself) combined with your username as a part of that url. After that SSO can pass to Discourse a field “invitedby” which will contain the same username.
But on second thought, there are too many parameters for it which are depending on an external system, so maybe it’s better not to implement it in Discourse itself, but in a plug-in.