Why use new FileReader reader approach to convert to base 64 when you could just use btoa()

Question

I am working on an application in react that uses an api and to make a post we have to send an image in base64 . I was looking on stack overflow and saw this example:

function getBase64(file) {
   var reader = new FileReader();
   reader.readAsDataURL(file);
   reader.onload = function () {
     console.log(reader.result);
   };
   reader.onerror = function (error) {
     console.log('Error: ', error);
   };
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file); // prints the base64 string

I'm confused to why I'd use new FileReader() when I could simply have one line of code that converts the string to base 64 using btoa()? Thank you in advance!

I can make this work either way I just don't know the point of using new FileReader() vs btoa.