An agent doesn't want to confuse a "fixer upper" with a "tear down." There is perhaps a fine line between them, but a tear down means don't put any value on the home. If anything take out the value (cost) of removing it.
Your questions seems some what ambiguous. I hope I can help answer this for you. If someone is purchasing a property that needs work and the work is too overwhelming or cost prohibitive to the individual that would be a case that a buyer or current owner would tear a house down as opposed to fixing it. In certain areas the price of land is more valuable without an old worn structure so that would be another case to tear down. Or perhaps the local code officials have deemed a structure uninhabitable and that would be another case for a tear down. Those are a few scenerios. I hope that helps. Best of Luck to you. Lucy