What is this poster there for? What is it saying? Why?

Why do i need to know that "WE ARE LONDONERS?" "WE ARE ONE" also?

Considering the cost of the advertising, printing etc. why bother filling the whole city with this statement? Who does it need to remind, or telling that? WHY?!?!?!?!?!??!?!?!

OK there is a sponsor name in the bottom left corner, who probably pays for the costs, but, why such a propagandistic old fashion kind of slogan?

I can only think of one thing: to create, or mantain a sense of belonging to a group, to being part of something and to make feel the reader proud of, more or less conciously.

In case of terrorist attack, or also while rising council taxes or transports fees, for example is this poster trying to say that after all the trouble we are "One", therefore struggling toghether we might feel a kind of relief? Let's notice that in the bottom right corner it also says "Mayor of London"...!