This on the Guardian blog answers the question of the function of Openers compared to other batters neatly
What is one looking for in an opener as opposed to a five or six? I
guess I have a rather simplistic understanding of the game. Thanks in
advance if you can shed some light for the less clued-in cricket fans
If the proper badgers will forgive a quick recap, then sure thing.
Basically opening the batting is the hardest job, because the bowlers
are fresh and the ball is new. This means that it is more likely to
swing, because the lacquer is still on the ball. It is more likely to
move off the seam, because the seam is new and hard. And it’s more
likely to bounce higher, because the ball is hard. As the ball gets
softer, it usually does less for the bowler. So an opener’s main job
is to survive, but some openers can be counter-attacking and throw the
bowlers into disarray by scoring quickly against the new ball.
Batsmen at around four and five are supposed to be the long innings
players who you build around, then six and seven can either save a
tough situation, or attack from a strong position, and can bat with
the tailenders effectively.