Answer to a) It’s not a functionality of Obsidian. It’s just a “use case” of links that many people resort to and that meets certain needs.
Answer to b) I dare to say that in 2023, it’s a commonly used concept in the PKM (Personal Knowledge Management) domain. Thus, the term “MOC” has become kind of a common word, in the same way as “PKM” itself, “TOC”, etc. It’s quite normal, if you’re new to PKM and to Obsidian, that you encounter certain terms you have never heard elsewhere. It will be the same in every field of the human activity.
Answer to c) : I don’t think there’s an assumption that everyone will want to use either the term or the concept. But it’s a fact that as soon as you’ll have more than a few hundreds of notes and these notes are supposed to constitute a knowledge base, you’ll feel the urge or the need to make one ore more notes collecting links pointing to related notes. If, instead of simply listing your links like in an index you write some words describing your links, your “notes pointing to a series of related notes” will inevitably look like an MOC. Call it “MOC” or not, that’s up to you. It’s as simple as that.