Seals actually go back as far as protoliterate Mesopotamia.; even before they were used to authenticate documents seals were used to seal doors and containers as a sort of indication that they had not been tampered with. So even without writing(and not everyone was literate nor would all literate people have handwritten their own documents as opposed to having a trained scribe do so) it was still a useful way of granting official assent to a document and certifying it as authentic. Seals were also valuable in other ways; they could be family heirlooms(as they were extremely technically demanding to make and usually made of rare and valuable hardstones) or amulets and they functioned as a kind of marker for legal personhood or at least citizenship. As for your question about forgery; that would have been fairly difficult to a degree since seals were handmade and to an extent customized so it would be very tricky to replicate on, but we do have some negative evidence of attempts in that Neo-Assyrian officials at least were authorized to demand further evidence or confirmation of any orders or letters they received if they suspected the seal or letter had been tampered with or forged. Loss or theft was another and probably somewhat more common concern; we have an outline for a legal procedure in which the owner of a lost seal publicly declares the loss of the seal and states that all documents created henceforth with said seal should be regarded as void.