When attaching multiple files to an email draft, it might be helpful to be able to rearrange the files in an order which makes sense. For example, you might upload 10 files that are a mix of excel files, PDF files, and jpg files. With this feature, you can quickly reorder the files by file type or by certain categories. Another example might be if you're describing the attachments in the email body and it would make sense that the files attached are in the same order as they are being described.
Currently without this feature, you have to attach the files one by one to achieve the same result. I believe it would be a nice QOL feature for everyone.