Currently, the first time a user on the Stack Exchange network reaches 200 reputation, they get an extra 100 rep on that account and all their associated accounts.
I believe this should be documented as a privilege at 200 rep (along with the lessening of adverts) for the following reasons.
When a user gets a new privilege, they get notified about it and would most likely go and see what that privilege allows them to do.
It would explain to new and active users that the SE site they are using - whether Stack Overflow, Super User or any other site - is not the only one. And that the community is larger than that, with a link to this page perhaps: http://stackexchange.com/sites
Also it would explain that there is no need to start all over again to be able to participate in any other SE site. And that they would already have some of the basic privileges.
One technical drawback would be the difficulty of what to do on SE sites where the user hasn't yet reached 200 rep, but has already received that privilege via another SE site?