Terran wall is a tool to fend off rushes and deny scouting. A good wall gives a huge advantage especially in lower leagues where rushes are prevalent and micro is abysymal. Many progamers do not wall, and rely on their micro which can end up with them losing to a forward gate Zealot or Zergling rushes as seen in recent ASL and KSL professional matches.
Map-specific doodads, cliffs, and neutral buildings also have their own gaps. It's hard to accurately predict the Building-Map Relationship, even on a new modern ladder map BlockChain.
Bottom right location on BlockChain
LOL, just one marine needed, suck it!
Bottom left location on BlockChain
A strategy used in Terran vs Protoss to fend off Zealots using micro, recommended for advanced players. Gaps are walkable by Marines and Zerglings, but not Zealots.
Minerals to the Left
Minerals to the Right
If your wall has holes, you can block them with just one SCV put on hold (You need to select SCVs together with other unit than SCV to be able to use hold position). Four repairing SCVs can keep a holding SCV alive indefinetely against a Zergling.
A Failed Wall
9 Pool? Bro it's BroodWall era time to change your strats
Just 2 SCVs on hold can keep Zerlings and Zealots on the ramp. The opponent can't manually attack because of the lifted Barracks, and units cannot get to Marine because of SCVs.
Click the video to play