Being aware of what your opponent is doing is key in all games, especially in bronze and silver level play. Most non-diamond league players will not even bother putting in the effort to kill your scout (Overlord, Observer). Knowing what your opponent is doing effects what you should be doing.
That first scout you send in can tell you more information than any other scout for the rest of the game. That first scout tells you effectively what you should be doing and preparing for.
If you scout your opponent and see 2 Stargates coming up pretty early on, then that means you need to build some anti-air, and fast. Terran always wall themselves in, so being prepared for a bio ball, or Viking push is a possibility, however to properly defend against it, you need to find out what.
If your Terran, then putting a well placed scan can help, if your toss, most Terrans don’t put up Missile Turrets unless they see air coming in so sending in an Observer can be a huge help, and if your Zerg, you should always have an overlord or two nearby keeping an eye on things, again most Terran do not put up Missile Turrets and most Bronze and Silver level players won’t make the effort to kill that overlord off.
It is important to remember that there is no perfect build that can hold out against everything. All strategies have their weaknesses. If your opponent is smart and scouts right, he will know what he has to do, and it is up to you to anticipate it and be prepared for that eventuality. It’s better to be safe in some cases than sorry. Always know what your limitations are.
The Xel’naga watch towers are a huge help in a lot of cases. They give you extended sight of what your opponent is doing before their units get in range to attack.
It’s better to know that your opponent is nearby than be paranoid that he is always right outside your field of vision. This is especially true against a Terran player using Siege tanks. The right time to strike would be when he is moving his tanks. Instead of attacking an entrenched position, you can possibly get close enough to hit his tanks or his units before the tanks have a chance to morph and get a shot off. Two armies of equal supply, one being Terran the other anything else will most likely lose to an entrenched Terran with Siege Tanks.
Suntzu said it, and it is a well known fact, as well as rule in a lot of strategy board games, “The defender always has the advantage.” Whether it is high ground or knowledge of an incoming attack, a player who is ready for an attack tends to win. That’s why you attack when he’s not. Watching and waiting are key. Watch him until he begins to move or get complacent, wait until he is in a vulnerable position, make sure he has no way of seeing you ahead of time, and then strike!.