Wizard of the Coast's recent announcements regarding changes to humanoids and alignment have spawned a fresh discussion of why we even have alignment as a factor in the game. Orcs used to be categorically evil; the new WOTC position is that all humanoids are basically unaligned (although they could certainly belong to an "evil" culture and play the roles of villains). For that matter, the 5E version of the game has dropped all of the player-character facing restrictions around alignment - paladins don't have to be Lawful Good, druids don't have to be Neutral, Assassins don't have to be Evil. What's the point of alignment in this new regime? Is it just a set of guidelines to roleplay your character? A mere descriptor of the character's attitude?
Here's the thing - regardless of how meaningless you think alignment is at the table, it is hard-wired into the cosmology of the game. Alignment describes the objective reality of the game universe. Each outer plane corresponds to a specific alignment and is home to a set of deities associated with that alignment. There are extra-planar races such as demons, devils, and angels that strongly correspond to outer planes and the alignments of those planes, too. Furthermore, when a mortal dies, their soul goes to the plane where their deity resides. Take a look at this picture of the 1st Edition Cosmology - although there have been tweaks here and there through the years (with 4E representing the biggest departure from the classic scheme) 5E has basically returned to the 9-fold alignment system and corresponding outer planes.
The 5E DMG goes on to say, "bringing someone back from the dead means retrieving the soul from that plane (the plane of the soul's deity and/or alignment) and returning it to its body". Regardless of how the individual players or DM fold, spindle, or mutilate alignment at their table, alignment is the basis for the transcendent reality beyond the concerns of grubby mortals. Players and referees may ignore alignment at the table, but it's there in the core assumptions of how the multiverse works.
Of course this is all governed by rule zero. You are the master of your campaign and game world, the creator of the cosmology. Your cosmology might deviate from the default assumptions and the rules as written. I get the impression many of the referees discard alignment as a relative matter, without considering that alignment has this objective game existence. At least if you're going to jettison alignment, replace the whole cosmology and redefine the objective reality of the game universe, too.
One can argue alignment is just a descriptor tag with no inherent meaning - a player calls himself "lawful good", acts arbitrarily, and the players and referee agree this aberrant behavior is actually what lawful good looks like for this particular cosmology. You've seen the argument, "I'm a viking, and lawful good to a viking means murder and pillage, amiright bro?" The important thing, from my point of view, is the relationship between the game objects - the "good" plane exists, there are "good" deities there, there are "good"-aligned extraplanar beings, some player characters pencil the word "good" on their character sheets, there's a recognition they're going to the "good" place when there's a character death. Ideally there's a common understanding of what is good or evil, law or chaos, too.
Alignment has bedeviled referees and players, but also sets D&D apart from almost every other RPG because of these behavioral guidelines and game universe tie-ins. Thus we grapple with questions in-game of what constitutes law, chaos, good, or evil and put on our ethics hats from time to time. My position is alignment is important mainly because of it's relationship to the broader multiverse and game fiction and less about managing behavior; but next time out let's look at some common approaches of dealing with how alignment can work at the table.