They died in a war where defensive technology outpaced offensive warmaking, resulting in the Race to the Sea and entrenched warfare, where only consistent brute force thrown at each side resulted in a stalemate. Without their sacrifice, Wilhelmine Germany would have conquered Europe and we would have had a considerably more dangerous and comparatively advanced Germany dominating Europe and invading Britain when compared to the threat we faced in WW2.
People too often deride WW1 as unnecessary. It was not. Wilhelm constructed war for the sole purpose of dominating Europe with an empire. There was no difference between him and Hitler beyond the anti-semitism and racial motives for the latter.
Although unfortunately biased, Max Hastings' documentary called 'The Necessary War' is by and large factual. I'd recommend giving it a watch. The sacrifices were for a good cause, and while mistakes by Generals occurred, it wasn't because of some madman tendency to throw away lives; it was simply because they didn't have the technology or experience to do anything different.