If knowledge is defined as justified true belief, then agnostic belief, if it is not an irrelevant proclamation of feeling, is simply justified true belief in a plausibility. If someone "believes but they're not sure," their belief can be equated either to knowledge of a quantified plausibility, e.g. "There is a 70% chance this is true," or a feeling, e.g. "I feel like this is true but I don't know." A claim of agnostic belief is therefore either equivalent to a claim of knowledge, or a proclamation of feeling that is justifiedly ignored.
So why the existence of the term? Well if there are no other possible reasons, the implicit conclusion is deception. The specification of "agnostic," by virtue of the apparent necessity of its specification, implies gnostic belief is possible and that one holds other, gnostic beliefs, which, by virtue of the presumed necessity of logical justification for a gnostic belief, implies there is logical justification for the specification of "agnostic," which could only be a quantified plausibility. The term is used to give this impression, and yet so that when pressed to reveal said logical justification, one can incorrectly and deceptively cite "agnostic," that they thus don't actually know anything, and thus have nothing to defend.
Mentioning one's own epistemology at all is just a big red herring; it has no place in debate; and it should only if ever be shared as nothing more than a cool factoid about one's self -- because there is no such thing as an agnostic or gnostic position. Whether one believes they can tell the difference between knowledge and belief is irrelevant. Neither is anything more than a statement of feeling. If someone says "I believe but I don't know this thing to be true," or "I know this thing to be true," either way the only proper response in debate would be "Prove it."
>inb4 someone contests that I can't disagree with "established" epistemology
Well, I can, and I don't. Nor does what you've read on wikipedia or from any atheist 'philosopher' constitute epistemology. The "science" is never "settled" so to speak, unless you're an idiot.