O'Neill is making a universal point about dreams and the power they have over us. It's not just the regular habitués of Harry Hope's saloon who dream their lives away; many of us in the world outside do too. To that extent, one can see the play's colorful cast of characters as a microcosm of the world in which they and we both live.
Everyone has dreams, and that's perfectly healthy and normal. But all too often they can turn into debilitating delusions, which prevent us from facing up to the realities of life. That's the condition in which Harry Hope's customers find themselves and from which Hickey wants to liberate them. But in due course even Hickey comes to realize that some people need their dreams—even utterly delusional ones—to keep them going through life's various trials and tribulations. The general point that O'Neill seems to be making here is that dreams are not just an optional extra but an essential part of what it means to be human.