In the first part of the macro, "/targetexact [Banner]" is there so that your target will not change to a random person with a similiar name such as "Mocker", or "Demoguy", when a banner is NOT up.
The exclusions in "/cast [noraid, noparty, noharm] Intervene" are designed so that you will not intervene a party/raid member, instead of your banner, if you are targeting (or your enemy is targeting) a party/raid member when the macro is pressed. As you know, its already impossible to intervene a friendly player out of group.
Finally, the exclusions in "/targetlasttarget [noraid, noparty, noharm, exists]" prevents your target from changing, unless the previous parts of the macro have been successfully executed against a banner, in which case, your last target is your original target, before you intervened your banner. This basically means that regardless of what you were targeting before you pressed the macro, you will still be targeting that target. Hope that helps!
PS: I know this macro is really more tuned for pvp, but please feel free to leave a comment! Are you warriors out there liking this?