As stated before several times in the thread, in real life you have moral ambiguity, mostly subject to a particular society or religious belief in particular and it´s part an individual choice, of course, but in D&D you need tags because mechanically there are a lot of spells, abilities and weapon damages that target a particular alignment. Also the creatures in the bestiary are catalogued within their particular alignment.
As @Killer Rabbit pointed out, in D&D the alignment axis are laws of nature, like gravity. In real life, you cannot launch a missile that targets only cheating husbands, serial killers or selfless philanthropists and left anyone else unscathed, but in Faerun you can.
From a game mechanics perspective, it´s interesting because it allows you to target yet another specific attribute of a creature, and that gives you much more options.
So unless you want chaotic/lawful weapons, protection from evil/good/chaos/law, holy/unholy word, etc etc to lose their reason-to-be; you need those tags because it´s a game with rules. The connotations of what that rules meant while roleplaying or what those alignments meant are, like many other things in D&D, subject to interpretation.
If you want to create another game that has different rules and use grey-zone morals that would be an interesting setting (Most TT games and fantasy books go in that direction the past years, including 5e. The alignment axis has very little relevance in comparison with past versions); but if you want a game that follows the rules of D&D 5e you need the alignment tags the same as you need the HP points, base attack or save spells of the creatures.
Yep, alignment is really one of the key pillars of DnD; whether or not it is likeable or realistic or easy to implement in a computer game, it is embedded in the rules ( at least, the old rules that I knew). Bioware did away with it for their DragonAge games, but then immediately needed to replace those "binary" worldviews ( law/chaos, good/evil ) with their own binary worldviews ( templar/mage, human/non-human ), which are far from nuanced, and possibly more annoying than the more abstract concepts of good and evil.
That said, I generally prefer rules sets that are less inclined to push choices on your character the way DnD does; In many respects I enjoyed Runequest more.
