I find alignment to be an essential part of D&D. It defines the cosmic forces of the multiverse, and connects them to the concerns of mortals.
Alignment, used correctly, is simply a labeling of in-world realities. 3e had the best presentation of alignment, and 5e's original presentation was basically a simplified (yet compatible) version of that.
That presentation of alignment never demanded that individuals were fixed to a specific alignment, outside of rare cases, such as celestials, fiends, undead, etc. Even those who were described as "always <alignment>" came with the caveat that once in a million exceptions happen. For everyone else, alignment had notes such "often" or "usually", with exceptions being common. Orcs for instance, were "often chaotic evil", and elves were "usually chaotic good".
That presentation of alignment had very few mechanical implications (and 5e reduced them further, which I generally approve of), but plenty of cosmological implications. In the Planescape/Great Wheel cosmology--what I consider the epitome of D&D identity--alignment tells us how the various locations of the afterlife function, and how one's behaviors will tend to determine where one ends up, and where one would be comfortable ending up. It explains story-rich possibilities for conflicts between various flavors of evil, such as devils versus demons, as well as philosophical disagreements amongst angelic sorts of beings.
It provides a cosmological canvas rich in potential in a variety of ways, but it is also useful and used on a regular basis in pretty much every session I run. The nine alignments provide a simple short-hand description of how an individual of that alignment is likely to behave on a couple of basic spectrums. If you tell me that an NPC is neutral evil, then even though there are a wide variety of different personalities of individuals that might be classified as neutral evil, it still tells me important things about that individual that I can use to adjudicate NPC decision-making. It's not a set of rules I have to keep referencing, but it's a quick and easy way to remember certain basic concepts about a particular NPC while I'm getting their character properly fleshed out and developed in my head.
This is exactly the same way it works for groups of beings within the D&D settings. It describes how the group functions. Like we might say the Scarlet Brotherhood is LE. That doesn't mean that all members of the Scarlet Brotherhood are LE, and certainly not everyone that lives in their lands is LE, but it gives us a quick mental image, in the same way as saying "they're like Greyhawk's Nazis" does.
D&D species are sometimes that way also,
almost always because of the religion centered on their creator deities.
For anyone, grognard or new blood, whose understanding of alignment doesn't include thorough familiarity with how it was presented in 3e, I refer you here:
3.5e Alignment. Since the
original 5e version presentation is closer to that than to any other version, you will better understand
that presentation by referring to 3e rather than older versions which haven't been official since last century.