Dear Auntie Gun Nut,
Why are the sights on my World War One-vintage Enfield graduated out to 2,000 yards? Was it because of trench warfare? Were these guys just amazing shooters, or what?
Most all of the rifles used in WWI were developed long before WWI. Besides, the enemy's frontline trench was rarely more than a couple of hundred yards away, and in some memorable sectors of the Western Front the other side's forwardmost trenchline was within easy grenade-tossing range.
The concept of volley fire dated back to the first gunpowder-armed troops. By the time of mass gunpowder armies in the 17th century, when the only way to hit someone with the smoothbores of the day much past 50-100 yds was with sheer dumb luck, battles consisted of you jogging all your columns around the battlefield to catch the enemy's guys, forming your columns into line, and shooting the other guy's line until it was softened up enough for a shock attack.
As rifled weapons came into general use, the military establishment completely overlooked their increased effectiveness in individual employment against point targets and said "Cool! We can deliver our volleys from further away now!" So, the American Civil War began with a display of classic tactics of the kind that had evolved in a nearly unbroken line from Maurice of Nassau to Gustavus Adolphus to Frederick the Great to Napoleon. When it became apparent that advancing in battalion columns against the massed fire of rifled muzzleloaders was a good way to get yourself shot to ribbons, troops on both sides began using tactics that would still be familiar today: pickets (LP/OP's), movement in dashes, et cetera.
A few years later, the Frogs and the Jerries decided to take a poke at each other. Despite the fact that troops of the French and Prussian armies were armed with vastly more modern weapons than the rifle-muskets of the American Civil War, they turned a blind eye to any lessons they may have learned from that first big industrialized conflict (indeed, Field Marshal Moltke scorned the very idea that there was anything to learn from "armed rabble chasing each other about in the wilderness.") The Austro-Prussian War and Franco-Prussian War were fought with weapons of the late 19th Century and tactics of the early 17th. When the tactics of Jena were applied to Sedan, the results were predictable and bloody. Flexible and forward thinkers that they were, the general staffs of the losing armies decided that their problem was (are you ready for this?) that their troops didn't charge into the massed, long-range rifle fire fast enough!
The end result was that the final rifles of the 19th Century were all set up for long-range shooting by fast-moving formations of infantry against other fast-moving formations of infantry, a situation that had only ever existed in wargames and in the minds of general staff officers...