The importance of the symbol of the unicorn in the uncut version of Blade Runner lies in the dream that Dekkard has of a unicorn and the way that the man who follows him and works for the police department leaves a paper origami figure of a unicorn. There is a definite correlation between them both. This correlation is actually foreshadowed in the film by the revelation that Rachel is a replicant who doesn't know she is not human. Dekkard reveals to her, rather callously perhaps, that all of her memories and dreams are actually nothing more than "implants." This of course raises the central question of the film, which is what defines a human and how does any human know that they are "real" and not a replicant with fabricated memories and a past that turns out not to exist.
The fact that Dekkard has a very vivid dream of a unicorn that nobody else could possibly know about presents the viewer with an interesting question. How could the detective who follows him know about it, unless Dekkard too is a replicant, and others know of his most secret dreams and past events just as Dekkard knows about Rachel's? The symbol of the unicorn therefore highlights the massive uncertainty that the film instills in the viewers. How is humanity defined, and how do humans know they are actually human? The unicorn is a mythological beast that does not actually exist. To what extent does "true" humanity fall into the same category?