As for what is he doing in that egg thing, I tend to go for the meditation idea, although the idea of the egg also being a sort of life support booth works too. Why can't it be both - he's meditating in a life support booth. As for why they had that scene in the movie, I'll go with Kersh's explanation, but Lucas' makes sense too. You have to remember that back in 1980, nobody really knew a whole lot about Vader. I suppose there could have been one or 2 people in the audience who thought he was a robot (and therefore his 'I am you father' speech would have seemed a bit odd).
As for what is he doing in that egg thing, I tend to go for the meditation idea, although the idea of the egg also being a sort of life support booth works too. Why can't it be both - he's meditating in a life support booth. As for why they had that scene in the movie, I'll go with Kersh's explanation, but Lucas' makes sense too. You have to remember that back in 1980, nobody really knew a whole lot about Vader. I suppose there could have been one or 2 people in the audience who thought he was a robot (and therefore his 'I am you father' speech would have seemed a bit odd).