I see a lot of terran replays and streams, and I see one common thing that I don't understand every time. When a Terran has mined out one of their bases, why do they build another CC for the next expansion instead of just lifting off one of the mined out expos? I understand making the first few, they need the energy for scans and mules, but once they've gotten 3 or 4, why do they continue to make them instead of lifting them off?
I think they just don't bother. Pros tend not to repair mech or burrow-heal roaches, and I don't think there's a good reason for these. They don't care about such things and prefer to spend their APM and attention on more important things. Also, after about 3 orbitals they tend to favour planetary fortresses for easier defence, which require seperate command centres (since all the previous CCs were turned into orbitals).
If 400 energy is good for scans and MULES, 600 is better right? However, I don't know for sure, I don't play Terran or competive.
Actually what Fenix said is true. In the lategame you can sacrifice scvs and replace them with mules so that you can increase your army value when maxed and that's proven to be a very strong strategy with terran... In the pro levels you can afford to not care to repair a hellion but you can't afford to spend 550 minerals to make a new orbital command...
Dropping several scans in rapid succession gives you a far clearer picture of the battlefield and foreknowledge of your opponents setup. Personally though I pefer to have Planetary Forces because they're devastating, especially with upgraded armor, range, and being repaired by SCVs - sometimes I build extra ones if I have maxed out forces and lots of resources.
in the end you want more units right? more Orbitals, means more production buildings, which means more units. you cannot support 10 barracks with only 2 orbiatl commands. When you lose one of your expansion,s and you need to quickly replace it, you can then use one of your originals CCs this is why they do it, plus it takes a while to lift off, i disagree with kuvasz, pros do bother repairing their mech units, and especially burrowing roaches...dunno what he's talking about. That's like saying pros don't bother using blink because it doesn't make much difference :/
I notice another thing that T tends to do is one base, I was playing a game and won probably because he didn't expand. I was just confused about that too