Why does when sending a file in multipart/form data in java we have to use both a Writer and a OutputStream?

Hi I have seen many sample codes working for sending a file in multipart/form-data in java. But they have used both Writer and an OutputStream. Why can't they use just use one of them?

Here is the sample code they have sent

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainClass_External2 {
    public static void main(String[] args){
        try{
            // Connect to the web server endpoint
            URL serverUrl =
                new URL("http://posttestserver.com/post.php?dir=example");
            HttpURLConnection urlConnection = (HttpURLConnection)serverUrl.openConnection();

            String boundaryString = "----SomeRandomText";
            String fileUrl = "abc.txt";
            File logFileToUpload = new File(fileUrl);

// Indicate that we want to write to the HTTP request body
            urlConnection.setDoOutput(true);
            urlConnection.setRequestMethod("POST");
            urlConnection.addRequestProperty("Content-Type","multipart/form-data; boundary=" + boundaryString);

// Indicate that we want to write some data as the HTTP request body
            urlConnection.setDoOutput(true);

            OutputStream outputStreamToRequestBody = urlConnection.getOutputStream();
            BufferedWriter httpRequestBodyWriter =
                new BufferedWriter(new OutputStreamWriter(outputStreamToRequestBody));

// Include value from the myFileDescription text area in the post data
            httpRequestBodyWriter.write("\n\n--" + boundaryString + "\n");
            httpRequestBodyWriter.write("Content-Disposition: form-data; name=\"myFileDescription\"");
            httpRequestBodyWriter.write("\n\n");
            httpRequestBodyWriter.write("Log file for 20150208");

// Include the section to describe the file
            httpRequestBodyWriter.write("\n--" + boundaryString + "\n");
            httpRequestBodyWriter.write("Content-Disposition: form-data;"
                    + "name=\"myFile\";"
                    + "filename=\""+ logFileToUpload.getName() +"\""
                    + "\nContent-Type: text/plain\n\n");
            httpRequestBodyWriter.flush();

// Write the actual file contents
            FileInputStream inputStreamToLogFile = new FileInputStream(logFileToUpload);

            int bytesRead;
            byte[] dataBuffer = new byte[1024];
            while((bytesRead = inputStreamToLogFile.read(dataBuffer)) != -1){
                outputStreamToRequestBody.write(dataBuffer, 0, bytesRead);
            }

// Mark the end of the multipart http request
            httpRequestBodyWriter.write("\n--" + boundaryString + "--\n");
            httpRequestBodyWriter.flush();

// Close the streams
            outputStreamToRequestBody.close();
            httpRequestBodyWriter.close();

            // Read response from web server, which will trigger the multipart HTTP request to be sent.
            BufferedReader httpResponseReader =
                    new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
            String lineRead;
            while((lineRead = httpResponseReader.readLine()) != null) {
                System.out.println(lineRead);
            }
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}