Magento is a vast platform and there are lots of other aspects other than programming. This is a dedicated platform where we can ask questions about anything related to Magento.
We were using stackoverflow in the past to ask questions about Magento before. But soon enough most of the questions which were asking in stackoverflow becomes out of topic. This is simply because stackoverflow is a dedicated platform for asking only programming questions. But the very nature of Magento is not so. Developers troubled with lots of other aspects such as how to fine-tune Magento, how to improve the speed and performance, how to fine tune cache etc. These questions are not suited in StackOverflow. This is what leads to the proposal of Magento stack exchange site by Ben Marks. The purpose is, the Magento community needs clarification on lots of subjects other than programming and hence we needed a dedicated platform for that.
That's how the history goes... After that, there was time, people continue to use StackOverflow for asking programming related questions and for other questions, they rely on the Magento StackExchange.
What can you ask here?
The answer is anything related to Magento.
Here is the on-topic link and what should avoid here link. Feel free to go through it and understand it.