The Mysterious Plasma Injectors

In my opinion, one of the most mysterious pieces of Star Trek technology is the plasma injectors. They are seemingly mentioned in every episode and movie of Star Trek, but exactly what their function is has never been pinned down. It has never been pinned down in any canon Star Trek sources, non-canon ones offer little more insight. The Star Trek Encyclopedia defines the plasma injectors as “a part of a starship’s warp drive” but does not state exactly what they do as to the functioning of that propulsion system. Besides this, the only other thing we know about plasma injectors is that they inject plasma, presumably the starship’s warp plasma, into something. So using this bit of information it is my personal theory that plasma injectors inject the plasma from the main reaction chamber to the warp power transfer conduits, or that they inject the energy produced by the matter anti-matter reaction into the warp plasma, “re energizing” it.
Of course a prerequisite to this would be that the plasma injectors were located very near to the warp core. However, in Enterprise, the plasma injectors are nowhere near the main reaction chamber or the warp power transfer conduits. They are located in a wall relatively far away from the warp core. Leading us back to square one – what in the hell do these plasma injectors actually do?