That way, resources will still be coming in to finance your war efforts, placing less effort on you to establish a successful expansion. If your expansion is shot down, at least the money from your original mining site can help you set up another. As a general rule, I try to expand whenever my resources start to pile up, usually at about the 1000-1500 mineral mark. People don't use specials. The great thing about Starcraft is that a big army doesn't necessarily guarantee you a win.
Whole armies can fall to even a small number of units.
Special Abilities are responsible adding the element of strategy to RTS games. All their opposing units were basically the same, not much variety. But it was the magic that had propelled Warcraft II into the RTS spotlight. The magic was responsible for adding strategy to the game, not to mention great fun and playability. The same notion can be applied here to starcraft.