Remember remember the
4x.sh4rp vs vOddy replay. If you haven't watched it, I strongly suggest you do, and pay special attention to the end game graphs...
Cost: $1500
Build Time: 15 secondsAnd that was in 1.08. Now that the economy has been nerfed, a strong economy is much more important.
A Scrin player will have access to very heavy units but they tend to be very expensive, meaning that late game spamming of units can be difficult. To compensate, there is no economy cap, and the Scrin can boost the growth of a Tiberium Field by 50%: a noticeable difference that will show itself clearly in a longer match.
The latter is of course done by use of the Growth Accelerator. They can be placed down over any Tiberium Growth Hole (they don't need Ground Control to be placed), and are built from the Defenses Tab, so if you've used the Fast Crane -> Ecoboom BO, you can use that Crane to build it while building up defenses from the Drone Platform.
This lends good use to the Tiberium Vibration Scan as well; if you have no units nearby, you can use it to find patches of Tiberium upon which you can place down Growth Accelerators, while keeping an early eye on your enemy's economy.
It is probably better to build them soon after you have a Nerve Center up, and before you get into the late game; this will make it easier to train up large numbers of the more expensive Tier 3 units, while preventing you from stalling your early-game economy.
Now you know what they can do, so why should you build them? Well, if you are facing a GDI player and have no Growth Accelerators, you will have thrown away a great advantage, as GDI cannot artificially increase their Tiberium Supply. In a Team game, placing these over yours and your ally's Tiberium fields will drastically boost your income over the course of the match. Against Nod, it will boost your economy much earlier than Nod can, and combined with Tech Center/Chemical Plant harrassment, ought to prevent your opponent from using Seed Tiberium often. Of course it goes without saying: defend them, repair them, and rebuild them as necessary.