Anything tha doesn;t correspond to wargear listed in the codex is pretty much there for style.
The Devastator box is definitely the most confusing though. Remember that a Devastator squad can have up to ten Marines in it. 1 Sergeant, 4 Heavy Weapon dudes, and then up to another 5 cannon fodder Marines. In gaming terms, you want those cannon fodder Marines around so that when the squad take wounds, you allocate as many as you can to the cannon fodder, so you can keep your heavy weapons alive as long as possible.
The point is, when making your cannon fodder guys, you've got two options. You can buy the combat squad box, and use those strange bits with the handles on them to model some basic marines that don;t look exactly the same as the ones in your
Tac Squads. The other option, if your
FLGS will let you, is to glue the skull and cable (Known as
Servo-Skulls in the fluff) guys to a base and have them count as a basic Marine.
I hope that clears things up a little. Happy to explain any other bits.