Walling is something Terrans do very well. The primary reason to wall is to help you defend as you tech. Walling is also capable of stopping early scouts, giving you an intelligence advantage.
Vs Terran:
Walling can be effective in TvT. It stops early rush’s by your opponent fairly effectively. There are a few reasons why it can be risky. Your opponent will have tanks. Early tanks means you will probably loose at least 1 supply depot. Remember that if your opponent doesn’t wall, he has a 100 mineral advantage on you. There is also a bug in the game that helps Terrans get through walls. Vultures can find a tight spot and lay a mine. With the mine taking up all the space, the vulture is pushed through the wall.
Vs Protoss:
Walls are more commonly seen vs Protoss. Early zealots or dragoons can be a real pain to deal with if the Terran player chooses to tech. apart from stopping pesky scouts, the wall also helps to deal with those issues by buying you time to get tanks created or mines researched. This is not to say you have to wall vs Protoss, but if rush's are hard for you to deal with than this becomes an option.
Vs Zerg:
Walling vs Zerg is almost never an option. early overlord scouts cant be stopped by the wall, negating the advantage of intelligence. TvZ is is very macro oriented. creating units should be your priority. the 100 minerals you spend on the depot could get you 2 marines or a faster second baracks. meanwhile zerg has created 4 zerglings for 100 minerals, and they can run through walls. so if you wall vs zerg be prepared for a hard game, and rather than using a supply depot to finish the wall, use a bunker to help keep lings at bay.
vs Random
Dont wall. If your oponent randoms terran or zerg it doesnt help you.
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