TLDR, POST has more stuff to it.

GET and POST are methods for HTTP Requests. The main ones of interest are:

• GET: get a thing
• POST: make a new thing
• PUT: change a thing
• DELETE: delete a thing

The most basic form of a GET is asking a specific URI to send back pre-set data. Also, GET tends to be the most common use request so most HTTP request-making methods will make this very simple. Thus, GET is often done with little more than a URI.

In contrast, POST has a lot going on.

1. You aren't making a GET request. This is more specific to Fetch API as it assumes GET by default so you need to specify in the options that you are using method: "POST".
2. You are, by definition, sending data. Therefore, you must include that data in the method: body: myData
3. Data has a format. The server has no idea if you sent it a string, and integer, a JSON, or the whole Harry Potter movie set. It just sees a stream of 1s and 0s. This is where headers come in, which let you specify to the server that you are sending it a JSON object or an image file so that the server can appropriately deal with the bits. Thus, we see headers: { 'Content-Type': <MIME type of myData> } in the options.