Qwerty, I don’t believe this is necessarily very important for outsiders to lubavitch to know and believe, I just don’t believe in hiding it when asked, because it’s privileged knowledge, not something to deny.
The Rebbe didn’t give reasons why he saw fit to gently let this out to those who cared to know, I can only speculate why.
I don’t think it has anything to do with the Moshiach campaign in the general sense: The Rebbe galvanized lubavitch about the urgency of bringing geula with the primary reasons: caring for the pain of the Shechina, down to very physical reasons such as life infinitely better than how we know it.
Nor did this knowledge endear Lubavitch more to others. So why?
After years of thinking about it, what I think may be the reason is because firstly, it makes Moshiach more real and imminent. This isn’t another campaign like there havebeen countless throughout Jewish history, which didn’t pull through. Nope, Moshiach has already been anointed, we’ve already entered the beginning of Yemos HaMoshiach (which coincides with the end of golus, so Halachically there’s no changes yet), and it’s going to pull through till the end, and is also dependent on our help.
Also, knowing that The Rebbe is Moshiach gives us encouragement with following his directives in preparing the world for Moshiach.
Every group has their emphasis and hadracha. Say the head of neturei karta was to be Moshiach, then helping pull down the state of Israel would be where to concentrate our efforts in bringing Moshiach for example.
Spreading the Rebbes message of a geula that includes and reaches every single Jew, Jewish pride as the way to fight anti semitism, seeing the world as an intrinsically beautiful place, and working on bringing that to the for more and more, spreading sheva mitzvos bnei noach, are all the Rebbes unique campaigns, and knowing he’s Moshiach helps us to have clarity and certainty with how we’re leading and teaching others.