In some ways, the Codec is simply a substitute for towns full of random NPCs a player can talk to. You simply choose an NPC to talk to and then listen to the NPC either give you some helpful advice or blab about something pointless, and is used primarily as a means of reminding the player of their current goal in the game. However, the implementation of the Codec system elevates it to a much higher level than just talking with NPCs, like what is seen in most other games.
First off, the most distinguishing aspect of the Codec is that the player can use it to talk to the support cast whenever the player wants, whether exploring a new area, sneaking around, or in the middle of a boss battle. So, the player can interact with the NPCs whenever the player wants to. The player can talk with the supporting cast whenever he feels like it, without having to go to a specific location or anything. Furthermore, this lets the player ask the NPCs for advice even during a boss battle. For example, if a player can't figure out how to defeat a boss like Vulcan Raven in the first Metal Gear Solid, he can always go to Colonel Campbell for a few tips. This makes the Codec characters generally much more useful than typical hint-dropping NPCs, since the player can turn to them for advice when he actually needs it.
And it really does feel like the player can ask the Codec characters questions. For example, if you equip a grenade in Metal Gear Solid 3 and call Sigint up, he will talk to Snake about grenades. Ditto for various other items and equipment in the game. This lets the player intentionally equip various items to provoke various different conversations with Sigint (or whoever else is giving this kind of information in other Metal Gear games), thereby letting the player "ask" him about things.
On top of that, the best part of Metal Gear Solid's Codec system is that the NPCs don't just give information to the player, they actually engage in a real conversation with the main character. Snake actually talks back to the NPCs, and they often go off on wild, yet very funny, tangents. A good example is how asking Para medic in MGS3 about any kind of animal results in Snake asking how good it tastes. Running jokes, conversations about pointless topics, and the like don't further the plot, but they do develop the characters. I have a theory that the best way to make a character likable is to have that character engage in pointless, mundane, silly, fun conversations. It is a great way to get to know characters.
A lot of the successful elements of the Codec system can be found in very different games. For example, I get a similar vibe from conversations in the Phoenix Wright: Ace Attorney series of games. Most of the time in any Phoenix Wright game, there is usually another character following the protagonist around. Whenever you examine something in an area, Phoenix and the other character usually end up talking about it. Furthermore, you really can ask NPCs about things by presenting them with evidence or asking about specific people. These questions also provoke involved, yet entertaining dialogues with the main character, that often involve running jokes and over-the top personalities. It is no wonder that the Phoenix Wright series and the Metal Gear franchise are well known for having memorable and well-developed casts of characters, even though they are very different games.