The Sreg service is used internally in the Mozilla infrastructure for security reasons and it is most likely that you will never need it outside Mozilla.
It’s used by the mozilla auth backend in server-core in order to separate system writes that can be done with the users credentials from those that require ldap admin credentials. This provides an additional level of security by keeping the auth credentials only on a box with limited access. The mozilla backend is used by the Authentication server and the Account manager server.
Note that some write operations (2) are not delegated, like changing the DN & the e-mail address of the user, so the Account manager will still bind for writes as the user.