What is the significance of the lines "Astride of a grave and a difficult birth...But habit is a great deadener" in Waiting for Godot?

In his essay on Proust, Beckett writes that habit is "the ballast that chains a dog to his vomit." This is clearly a criticism of habit. Habit, by itself, is this idea that we repeat the same thoughts or actions simply because we have become used to doing so. In this sense, habit is thoughtless and robotic.

In this play, the two main characters continue to wait for Godot. They waste vast amounts of time...

In his essay on Proust, Beckett writes that habit is "the ballast that chains a dog to his vomit." This is clearly a criticism of habit. Habit, by itself, is this idea that we repeat the same thoughts or actions simply because we have become used to doing so. In this sense, habit is thoughtless and robotic.

In this play, the two main characters continue to wait for Godot. They waste vast amounts of time waiting for someone who is (likely) never going to show up. They waste large portions of their lives waiting. A life is between birth and the grave. Each person is basically "astride" birth and death, one foot in each. We stride from birth to death. So, to spend that "stride" waiting for nothing, simply out of habit, is a waste of a life. In this passage, Vladimir is making complete sense: habit meaningfully kills a person by making him/her waste time with empty habits. 

The problem, as Vladimir sees it in this passage, is not a lack of time. The problem is that humans spend too much time with pointless habits. There is the added notion, derived from the play as a whole, that it is also useless to depend upon some outside authority figure (Godot). If Vladimir and Estragon could assert their own free will and their own independence from Godot, they could stop waiting and start doing more significant things with their time. They need to break from this habit of waiting. Breaking a habit or breaking a tradition allows one to experience new things. This seems like an obvious statement, but everyone is guilty of repetitive behaviors and ways of thinking. When these repetitions (habits) continue and produce nothing meaningful, the habits become "deadeners."