These films are rather cheesy of course. The acting, largely the result of actors of that era being trained for stage and silent films, is poor by our standards, the effects far outdated, and the story lines are often rather weak - although I confess I find these films endearing for these reasons.
But these films have shaped what Halloween monsters are. The Draculas and Frankensteins we see trick or treating every year are the versions from these films. They have had a remarkable cultural impact. There are a number of factors that explain their enduring appeal.
These films are rather primal, touching basic fears and struggles.
First, each of the classic universal horror films deals squarely with death, fear of death and longing for death. The lines are a bit campy, hence Bela Lugosi's Dracula tells us "To die, to be truly dead ... that must be wonderful," and "there are far worse things awaiting man than death!" But most films of that or any era could not face death so head on. The fact that these are "monster films" gives them free leeway to actually explore our struggle with mortality.
Second, the power and danger of sexuality is strongly dealt with. Whether it's the Wolfman's desiring to "devour" his love interest, or the strongly implied lesbianism of Dracula's Daughter (an odd, but must see film), sex is presented as something bubbling below the surface. The characters try to control their libido, attempt to master it, but all too often fail as the force of their passions carries them to a dangerous place.
Third, there is a fear that science and technology may harm us. We hear again and again in these films that "modern science" does not permit the existence of this or that monster. Most tellingly, in The Mummy we are told that one must violate ancient graves because of "science you know." Each of these films fears that we have lost something ancient and essential because of our advances, and we are in real danger for having lost it.
Fourth, and finally, the classic horror films question and examine the notions of "normalcy" and "the other." In some, e.g. Dracula, "the other" is dangerous and must be destroyed. In many others - see Tod Browning's Freaks especially - , e.g. Frankenstein, the other is simply misunderstood and it is the "normal" people who are dangerous, vile, and destructive. The angry mobs in Frankenstein and Bride of Frankenstein are eerily reminiscent of today's "Tea-baggers" who fear shout and hate.
Because these films are "horror" or "fantasy" they explore issues that more mainstream films at that time did not. They remain worth watching for this reason. They look at issues we must still examine, they wrestle with conflicts we still struggle to understand.
So, do yourself a favor and watch some of the classic Universal Horror films this Halloween.