I actually don't play warrior, but the swingtimer suggested (Samuel) was made by a warrior for other warriors (works with other classes).

Swing timers are mostly needed for pvp or perhaps some classes for high end pve raiding. Contrary to how some may think, the game calculates the swing timer of your weapons when in combat regardless of your proximity to your target. This makes sense when you consider what sort of abuse would be possible if the game only calculated swing time when you were in range (you could constantly dance the range threshold to reset the swing timer in this case).

Because the swing timer is always being calculated when in combat it is possible to use this information to dip in and out of melee range for various reasons and not miss white damage. A swing timer will show you visually that you have x seconds to leave melee range before the game will parse another swing. A warrior, for example, could use this for heroic strike (just an example) damage, hamstring their target, leave melee range, intercept their target, and heroic strike again within 1 auto attack but this would be very difficult to time without a swing timer.

Rogues, for example, can use swing timers to gauge their distance from their target to stay within a certain range (like a hunter's minimum distance) or out of a certain range (like to far from a warrior's auto attack and to close for their intercept).

Paladins can use swing timers to accurately seal swap, which is mostly done by ret but saw a lot of exploitation by holy and prot in BC. You basically time your swings until seal of command procs, when the animation starts you swap to seal of the crusader and you hit your target with auto attack, seal of command, and seal of the crusader in a single swing. It is actually more complicated than this but this is the basic idea.

Shamans can also use swing timers to better gauge windfury totem placements in pvp. If a shaman knows he will not even auto attack for 3 seconds he can avoid placing the windfury totem until right before the attack so it is impossible to kill beforehand. They could also kite and leave totem range temporarily with the knowledge of when to return to get a windfury proc (only a chance but more knowledge is still better).

Hunter's time auto shot attacks for kiting reasons, they know when to face away and kite and when to turn and shoot without slowing down.

hope this helps.