What is the merger principle in law and what is the effect of it?

Lord Denning amplified on the rationale of the merger principle in Ward v. Lewis, above, at p. 56: The prior agreement merges in the tort. A party is not allowed to gain an added advantage by charging conspiracy when the agreement has become merged in the tort. It is sometimes sought, by charging conspiracy, to get an added advantage, for instance in proceedings for discovery, or by getting in evidence which would not be admissible in a straight action in tort, or to overcome substantive rules of law, such as here, the rules concerning republication of slanders. When the court sees attempts of that kind being made, it will discourage them by striking out the allegation of conspiracy on the simple ground that the conspiracy adds nothing when the tort has in fact been committed.