I'm not really a noob ARC/BRD but ...

[Myon88]

It's basically free damage if you use it, as Krindor mentioned, because of the way you can fit it between weaponskills.
If you do nothing but spam Heavy Shot for instance, your damage going out will look something like this,

[Heavy Shot][-----][Heavy Shot][-----][Heavy Shot][-----]
2.5s---------------2.5s----------------2.5s

Because the cast time of Heavy Shot is less than the global cooldown, there's downtime after the cast where you can't fire any other weaponskills. If you use that opportunity to fire an OGCD like bloodletter it will look more like this, and you will be dealing more damage.

[Heavy Shot][--BL-][Heavy Shot][-----][Heavy Shot][-----]
2.5s---------------2.5s----------------2.5s

Using OGCDs like that is generally called 'weaving' since you are fitting them between weaponskills. As a rule of thumb, if you cast a weaponskill manually because of minuet, you can only fit 1 OGCD behind it before your GCD is complete and you should cast your next weaponskill. If you manage to fire a weaponskill instantly (straighter shot proc, or you turned off minuet for some reason), then you can fit 2 OGCDs behind it ('double weaving').