Why and When to use JSON

I know what JSON is and what the advantages over XML. I already read some answer for this , but still i can't get through it.

So i would specifically ask this questions:

1. Is it only useful for API thing? so exchange data without refresh the whole page using AJAX..

2. Is it always used with AJAX?

3. Do people (always/very often) using JSON like this? :: Database/Server - JSON - Client.. what i mean by that is, all our data from database will be put into JSON, so people can use it easily?

big thanks everyone!