Visualization has been a tool of trainers and champion athletes since the 1930s, when the Germans started using it.
And visualization can take many forms. Visualizing an opponent helps. Visualizing yourself deliver crushing blows and wearing his ass out help. Visualizing good technique, or visualizing good movement (I've improved my punching by visualizing my torso as a coiled spring, which I use to spring my fists forward) helps.
Employed properly it can help develop good technique and good attitude, the latter being important in wining fights whether they be in the street or the ring. The mental aspect of your game helps as much as anything; that's what makes champions champions. Their perception is that they're the baddest motherfuckers in the fight, and the perception becomes a self-fulfilling prophecy.