Purposes of Editing

Why is editing needed?


Creating pace
Generally speaking, shorter scenes are read and understood more quickly by the audience and make for a faster moving sequence, such as action scenes. Note the length of each scene in this example of the fight scene from Avengers: Age of Ultron:

Continuity

Editing is sometimes used to keep continuity in a sequence. This is to make sure actors are wearing same cloths, in same position etc

Removing Mistakes

This a bloopers or scenes showing mistakes taking from the second series of Mrs Brown's Boys.

Selecting the best take
Editing often gives directors and actors several chances to choose the best version of a scene

Censorship
Sometimes editing is needed to remove a scene that would  have the movie blocked by the BBFC - a famous example is 11 seconds from the Oscar winning Movie Last Tango in Paris had to be taken out at the last moment

Reducing a film or sequence to a preferred length
Sometimes a film has to be edited down to a shorter version to be taken into distribution. Here is an unused scene from Snow White and the 7 Dwarfs: