All it does is soften the decal carrier film, and break the surface tension between said film and the receiving surface, so that the whole decal area conforms as smoothly, and as closely as possible to that surface. In doing this, the decal carrier film will 'stretch' (for want of better description) and 'bed down' to be almost invisible. The 'quality' of the end result is dependant on the quality of the surface to which the decals are applied, with a smooth (ie gloss), grease-free finish being required to obtain optimum results. Once this overall surface is 'equalised', over the decal area, by application of a further gloss coat, then the results are complete, with the decal(s) now being bonded to, and part of, the surrounds,.
Further clear coats of the required finish (ie, matt, gloss, semi matt etc), then enhance the appearance, giving the visual perception of the decal(s) being painted on.