Since you did ask, and since you appear not to have been trained in a USMC military police school, I will elaborate.  But in fact I already included the explanation:  the uniforms set expectations.  Expectation fulfillment is probably the most powerful tool in the shed of a person who wants to keep the peace.   Do not crowd a person - give them space not to feel threatened.  Do not dress, speak or carry things in ways that imply you intend to use force.  Instead, model the behavior you wish for.  Speak slowly, rationally.  It is amazingly powerful.  I have used it inside prisons - where staff must be unarmed and deal with people who - well - need to be in prison.  One inmate, a giant towering over me, once declared:  "I don't know why I let you talk to me like that?  I don't let anybody talk to me like that!"  The wrong time to explain expectation fulfillment, I decided.  I also was not afraid - did not telegraph fear - because I had options.  He was dismayed by non victim behavior.