Something that has seemed pretty clear to me from the moment I began learning about World War I in school was that there was really no point. Weapon technology, human population, and alliances between nations had all blown up, so it's no wonder that one of the earlier post-industrial wars was just a travesty for human life. All that seems to really have been gained was a change in the essential way that war worked, as civilian slaughter became more and more normalized, with thirteen million civilians being killed, more than the still shocking 8.5 million troops. As airplanes had only recently come into existence, it was the first war to be fought on land, air, and sea simultaneously. It really served to mark a turning point in history when it comes to how war was fought. In addition to massive loss of human life, and the emergence of an acknowledgement of psychological stress such as PTSD ("shell shock"), the scars left by the war were permanent, as the experiences of those who fought in it are still infamous to this day for being more harrowing than most other wars throughout history.
One of the most important lessons to learn from the war, to me, is that governments can very effectively use propaganda to get their citizens to do anything, even die for their country. Watching They Shall Not Grow Old, something that struck me is that most of the men giving accounts of their time during the war said that they felt excited and exhilarated upon going out to fight, instead of scared or at the least sort of apprehensive. They believed that what they were doing, fighting for their country, was something that they genuinely wished to do with their lives. There are tons of examples of government propaganda from the war, such as the United States referring to the German as the "Hun", or another poster promoting warfare by stating "It is far better to face the bullets than to be killed at home by a bomb", and it's clear that it was extremely effective. When effectively used, propaganda like this is capable of convincing people to do absolutely anything in the name of patriotism, and it's important to remember how untrustworthy it can be, given the juxtaposition of the excitement before going off to fight and with the real horrors faced during the war.
World War I was, obviously, not the first war in which civilians were killed, nor was it the first war in which civilians were killed en masse (we've all heard of the atrocities in the Herero-Nama genocide about a decade prior). Still, it feels like it marks a turning point in just how many civilians were massacred. This was normalized during the war for the purpose of, among other things, curbing enemy morale during times of war. It was the first time air attacks had ever been used on British civilians, or civilians in most countries for that matter, and with new technology such as powerful bombs, poison gas, and flamethrowers, it's no wonder that people were so eager to kill non-combatants. It's hard to say that war could ever be the same again after such an all-encompassing war killed so many innocent people and used so many new weapons, and by now we all know that those tactics and developments and weaponry did not slow down in the following decades.
One of the most important reasons to understand World War I is that it serves as an extreme example of a war being fought for no good, valid reason, and leaving scars on those who participated. That being said, it doesn't do any good to write it off as some comically useless fight and completely ignore it, because it could happen again. Obviously it would be fought differently, but we still have complex systems of alliances between different countries and even more devastating weapons that we did in the early twentieth century. Fundamentally, it's important to be able to understand how governments can rouse their people up to do these things in order to make sure that people don't fall for the same propaganda again in the future.