Art and activism

Lʹart is very present in social movements. It expresses itself through poetic slogans, choreography, installations, graffiti or collages on walls. How can we explain the role art plays in activism? Is it the most effective way of getting a message across? Does it reveal a need for beauty in what can be a harsh struggle? Conversely, many artists do engage in lʹart engagé. Does art have to be militant?