As for the effectiveness of snipers, one of the most powerful effects from sniping, and what makes it such a devastating tool in a military arsenal, is the psychological repercussions it introduces into the enemy psyche.
Sure, if you are the one actually struck by a sniper's round, it is a gonna be lot more than just psychological effects you feel, but for the ten+ guys standing nearby just all shooting the shit with you at that time, when you get hit and your head gets turned into a canoe by a high-powered rifle round out of the blue delivered by a sniper from far off and you drop dead in front of your buddies, those ten+ guys are going to feel some serious psychological effects from seeing that. They are now gonna be keeping their heads down in justifiable fear, and will constantly be aware that at any given moment, a single well-aimed shot coming without warning (sometimes coming from extreme distances) can kill them just as easily as it killed their friend.
This is the primary reason, really since the dawn of the rifle, that snipers have been universally hated by opposing forces. Regular infantry soldiers typically HATE enemy snipers even moreso than they do regular enemy infantry for example, and many consider the art of sniping to be almost unfair, unsporting, or lacking gallantry or bravery, as in many cases the sniper is shooting someone indiscriminately with a single shot from stand-off distance in cold blood while the targeted individual thinks they are actually relatively safe at that moment. Having someone on your side become the victim of a sniper can be morale-destroying for an entire group of soldiers, and a single sniper kill can force large groups of soldiers to have to alter their defensive tactics remarkably and make them constantly fear for their safety (even more than normal) if they expose themselves for even a second or two. That negative psychological effect on large groups of men is where the primary effectiveness in sniping tended to lie during WWI (and beyond).
In WWI, if you stuck your head just a bit above the lip of the trenchline, a waiting enemy sniper might just put a well-placed bullet into your skull and end you in the blink of an eye from a long distance away. It certainly happened many times in that conflict on both sides, and it had a brutal effect on morale throughout the trenches.
WWI was particularly well suited for sniper tactics. This was due to the static trench warfare strategies employed by both sides that kept the enemy in a relatively confined known stretch of area near the front lines. If you exposed yourself by raising your head above that trench line, you had to expect you were potentially in the line of fire of an enemy sniper who could strike with deadly accuracy from anywhere in front of you.
WWI also saw the introduction of the use of sniper-spotters - who were observers who would many times accompany the actual "shooter" with the sniper rifle, where these observers would act as "spotters" (a second set of eyes) to increase the odds of locating an enemy target of opportunity to engage. The spotters would many times carry a periscope or periscope-binoculars, which would allow the spotter and sniper to remain safely behind cover while still allowing them to scan for targets. Once the spotter ID'ed a target, he would tell the sniper where to look, and the sniper could then pop up and quickly fire at the target in that location before ducking back under cover once their round was sent downrange (snipers and their spotters were well known in WWI for tending to have a very good knowledge of enemy trench formations and locations, allowing the spotter to quickly convey targeting location data to the sniper so the sniper knew exactly where to aim when he popped up). The spotter would constantly scan the enemy trench line with their telescopic periscope, looking for an enemy head or helmet peeking above the lip of the trench, or for an enemy soldier exposing himself briefly in a crack in the defensive line.
Many WWI snipers did not actually ply their deadly trade from the trenches proper. Many instead, along with their spotters, typically preferred to crawl out into "no man's land" (usually under cover of night) and, while well camouflaged and preferably behind cover, would remain motionless, lying in wait, many times for many hours at a time, looking for an opportune kill shot to present itself.
Being a sniper in WWI was a dangerous role. If captured, a sniper could typically expect very little quarter or fair treatment from the enemy (due to the psychological reasons I outlined earlier in this post, snipers were typically treated very harshly if captured). If they were sniping from "no man's land" and their location was detected by the enemy before or as they fired, snipers could expect an artillery barrage to be called in on their location, with shells raining down upon them in short order to wipe them out.
Snipers in WWI proved to be highly effective at not only killing the enemy, but also just as if not even more importantly, at destroying enemy morale throughout the enemy trenchline. It was a dirty, dangerous job that required a large degree of skill with a rifle, making snipers some of the most feared of specialist soldiers on the WWI battlefield.
