As a filler it can be used when you either have nothing else to add, do not wish to say anything more, or when you do not wish to state the the obvious.
Also quite useful as a conversation ender when you're talking with someone unfamiliar or simply undesireable.
person 1: Yeah I like that band even though you hate them, you just don't have to come to the concert with me
person 1: Alright then, I'll go with suzie.
(You in this situation are upset by the fact that this person is rubbing it in that you hate their shitty style of music so you wish not to say anything stupid in place of mern ie.: "you are an idiot for liking that" or state the obvious: "Yeah, I don't like that shit". However this situation is also unique in that you also have nothing else to add, this person knows you don't like it and thats been said, you don't even like concerts and they know it, changing the subject is inappropriate here, so you have no choice, but to use "mern".
as a conversation ender:
person: Hey, havn't seen you in a while, it's like you disappeared. You've been like ignoring my texts and stuff.
you: Yeah, well mern.
(obvously the person in this example is undesireable because they use phrases such as "like" and "and stuff" also clearly you are ignoring them probably because of their social ineptitude or a variety of reasons. Notice how there is no reaction from the person because they are unaware how to proceed after such a statement. As they are both confused by the meaning and use of mern they have no response.)