We don't need it, just like we don't need medicine. But it's sure as hell nice to have. So I want to focus on ML (machine learning) rather than AGI (artificial general intelligence).
ML has opened up a lot of options for us. Not only is it used to help select items that we might like when shopping online, but it's also used in medicine. We can take massive volumes of data and let a computer figure out patterns in order to make predictions about drug interactions, matching drugs to patients, diagnosing conditions, and so on. And the better our methods get, the more useful it'll become.
ML and AI are also used in automation. We'll need it for autonomous vehicles. AVs are the future of transportation. Just think about being able to put a truck into auto pilot so you don't have to worry about a driver passing out behind the wheel. Or imagine no more drunk driving. Sure, it'll be disruptive, but so what? The benefits are clear.
This is a typical "it depends" kind of questions. There are areas that require AI whiles others do not. For example, understanding human emotion is an area that is best not left to an AI. An area that requires AI is mechanical problem diagnostics. Hence the reason why I say this is an "it depends" question.
The answers to the next two questions is a different story. There is still ongoing debates as to best to address these two questions.
We as humans are constantly looking to improve things, be it our own situation or other's situations. However we are limited being humans, we cannot function as fast as a computer on specific task. Therefore we are moving to use AI to make computer more capable than ever at problem solving. The effects on humanity however are relative as a singular person may not benefit much from AI but as a society we can use it to help improve infrastructure and global situations.