Comment 6 for bug 556347

The help now says:
Purpose: Graphically visualise one or several branches.
Usage: bzr visualise [LOCATIONS...]

...

Description:
  Opens a graphical window to allow you to see branches history and
  relationships between revisions in a visual manner,

  If no revision is specified, the branch last revision is taken as a
  starting point. When a revision is specified, the presented graph starts
  with it (as a side effect, when a shared repository is used, any revision
  can be used even if it's not part of the branch history).

The purpose is to get a quick representation of the differences between two or more branches, is one merged into another, when did they diverge, etc.