Why is agriculture important in the survival of mankind?
Agriculture is important in the survival of humankind because it allowed more people to exist on the Earth. It is not clear that this makes human beings as a species more likely to survive for a long time, though.
It is not as if people cannot live without agriculture. There were human beings for tens of thousands of years before agriculture was invented. People are perfectly capable of making a living through hunting and gathering. However, hunting and gathering limits the potential size and density of human population. Hunter-gatherers typically have to live a nomadic lifestyle and cannot support large populations. They also cannot have much in the way of technology.
Therefore, agriculture makes our modern way of life possible. It makes it possible for civilizations and technology to arise. This allows more people to live on the Earth at any one time, but it is not clear that it helps human beings as a species to be more likely to survive for a long period of time.