It's all about REST semantics.
POST basically that you are posting a request which is going to modify the server state.
POST index/type
{
"foo": "bar"
}
will generate an _id server side and will index the document with this _id.
PUT is used to send a resource to the server.
PUT index/type/id
{
"foo": "bar"
}
will put or update a document named index/type/id in the server.
Makes sense?
More details here: https://en.wikipedia.org/wiki/Representational_state_transfer