ME: "lol good game man, u need to work on ur defense"
OPPONENT: "i know, i rly suck against tanks"
ME: "lol do u have any other buildings or something?"
OPPONENT: "maybe...u need to scout hehe"
So I search bottom right before looking at top left, where he apparently already has about 5 cannons, 4 gateways, citadel, templar archives, 2 stargates and a fleet beacon.
I move my forces up as fast as I can, but in a moment of inattention, my vultures die to dark templar and my tanks die to his speed upgraded zealots. I expo, lay down 2 more factories, and macro tanks and goliaths as fast I can. Not having expected the hidden base and the switch to carriers, I start the goliath missile range upgrade.
But somehow, no matter what I do, I can't win any of the battles. His carriers, zealots, goons, dark templar and templar+storm combo crushes my tank and goliath armies every time. It didn't help that I'm d- and my opponent was at least c.
...
So anyway, it seems pretty clear to me I would have won if I just could have laid a mine at every possible expo location.
Therefore, I think just laying down a mine (as terran), burrowing a ling (as zerg) or putting an observer (as toss) at expo spots is worth the investment. It helps to avoid unexpected surprises