It allows you take make other mistakes, such as using the wrong type for POST data resulting in "[object Object]" and similar gibberish being sent to the server.
Not saying XMLHttpRequest API's is brilliant, but fetch's API also doesn't exactly fill me with joy.
I think the key is the comparative "-er" in "cleaner" and "simpler." Fetch may not be the cleanest or simplest possible solution, but in my opinion it is cleaner and simpler. XMLHttpRequest is good. Fetch is better.
Not saying XMLHttpRequest API's is brilliant, but fetch's API also doesn't exactly fill me with joy.