Basically, what this does is getting a focus-free Aimed Shot thanks to the Master Marksman effect.
At 4 stacks of the Master Marksman effect, if you manage to get lucky and cast a Aimed Shot after the Steady Shot that procs the Fire! buff, your currently cast Aimed Shot will be focus free but doesn't clear the Fire! buff. This way if you keep check of the buffs, it's advicable to switch to casting Aimed over Arcane Shot in raid situations where you see you have 4 stacks of the Master Marksman effect.
Also has funny effects in Arena, if you're lucky enough to get this with Rapid Fire on, you can chaincast 3 Aimed Shots within 4 seconds. Yes, that's a lot of damage. Honestly, this rarely ever happens in PvP, but in PvE this is a viable exploit to pay attention to.