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First, what does read magic do? Here is the text (Men & Magic, page 23):
The means by which the incantations on an item or scroll are read. Without such a spell or similar device magic is unintelligible to even a Magic-User. The spell is of short duration (one or two readings being the usual limit).
So, read magic is the "key" that fits the "lock" of scrolls (most commonly) and even perhaps other magic items. I take that mention of "an item" to be a great suggestion to put instruction runes on all kinds of magic items (and even architectural features). As Talysman writes in his spell series post on read magic:
The Read Magic spell appears to have originally been a "gatekeeper", blocking immediate access to something; in this case, magic scrolls or activation inscriptions on magic items. You can decipher magical inscriptions later, at great expense over several weeks, or you can cast Read Magic now -- but that means devoting a spell slot you might prefer to use for something like Sleep. The intention, then, was that scrolls found in the dungeon would normally not be usable until much later, with Read Magic allowing you to bypass that rule.That's what makes read magic as a spell interesting. If you don't have information about the dangers and obstacles that you will be facing, then picking your spells is much like rock, paper, scissors. Once you have done some reconnaissance of your target, you are then operating with more information, and can react appropriately, but that comes at the cost of time and perhaps alerting the denizens of the dungeon to your presence (an aside: this is also why the "15 minute adventuring day" is a feature, not a bug).
This is also an argument against including spells like magic missile, especially on the low-level spell lists. It's not a strong argument, as the obvious rejoinder is that not all obstacles can be solved by combat (for example, read languages might be more useful if you need to figure out how to activate an ancient machine). But magic missile does lack some of the interesting trade-off inherent in a spell like sleep, which is incredibly powerful against enemies like orcs or low-level humans, but useless against undead. Magic missile is also less interesting because missiles are how fighters solve problems. Even fireball and lightning bolt don't really have that issue within the structure of the game, as they attack enemies in novel ways (by area of effect and line of effect, respectively). [Edit: see also Talysman's post on magic missile in response.]
A number of other spells plug into other aspects of the game in ways that may not be immediately obvious. For example, read languages is not just about translation; it is the "key" that fits the "lock" of treasure maps (Men & Magic, page 23):
The means by which directions and the like are read, particularly on treasure maps.Another example: charm person is not just about getting your way with some NPCs, it is also a way to obtain a retainer while bypassing the negotiation step and the reaction roll social mechanics. From Men & Magic, page 12:
Monsters can be lured into service if they are of the same basic alignment as the player-character, or they can be Charmed and thus ordered to serve. Note, however, that the term "monster" includes men found in the dungeons, so in this way some high-level characters can be brought into a character's service, charisma allowing or through a Charm spell.
Thus, charm person is more properly understood (in terms of its place in the game) as a more reliable replacement for a high charisma. Almost every single magic-user spell in the 3 LBBs has this quality of being scissors to some situational paper.
This principle almost immediately starts to break down with later versions of the game, however. For example, Supplement I: Greyhawk includes the first level spell ventriloquism. Now, it's true that use of ventriloquism could lead to some creative problem solving, but it is not clear to me how it fits into the structure of the game in the way that most of the original spells do, and thus also unclear why a magic-user might want to make the trade-off of preparing ventriloquism rather than some other spell.
Just like the equipment list, the spell list is potentially a valuable source of information regarding the types of problems that will be present and thus what the game is about.