Multipart is intended to respond to browser requests. Even though you can of course create any request using THttpClient (just setting the proper headers and raw byte content), when using THttpClient you have more flexibility to send data to the server, so you can use easier method for you. Is there a reason for using multipart format?
That has more to do with the THttpClient mechanism than multipart itself. Indeed THttpClient requires you to provide all the request body before sending it to the server - regardless if it's multipart or not.