libssh2 non-blocking sftp write example: waisocket() called only in specific cases

Question

So I studied libssh2 example about how to implement a non-blocking sftp write operation (https://libssh2.org/examples/sftp_write_nonblock.html), and I noticed the following difference when checking the return value of a non-blocking function call:

waitsocket() is not called in case of libssh2_session_handshake(), libssh2_userauth_password(), libssh2_userauth_publickey_fromfile(), libssh2_sftp_init(), libssh2_sftp_open(). For example:

    while((rc = libssh2_session_handshake(session, sock)) == LIBSSH2_ERROR_EAGAIN);
    if(rc)
    {
        fprintf(stderr, "Failure establishing SSH session: %d\n", rc);
        goto shutdown;
    }

but it is called in case of libssh2_sftp_write():

while((nwritten = libssh2_sftp_write(sftp_handle, ptr, nread)) == LIBSSH2_ERROR_EAGAIN) 
{
    waitsocket(sock, session);
}

I add the implementation of waitsocket() below:

static int waitsocket(libssh2_socket_t socket_fd, LIBSSH2_SESSION *session)
{
    struct timeval timeout;
    int rc;
    fd_set fd;
    fd_set *writefd = NULL;
    fd_set *readfd = NULL;
    int dir;
 
    timeout.tv_sec = 10;
    timeout.tv_usec = 0;
 
    FD_ZERO(&fd);
 
    FD_SET(socket_fd, &fd);
 
    /* now make sure we wait in the correct direction */ 
    dir = libssh2_session_block_directions(session);

 
    if(dir & LIBSSH2_SESSION_BLOCK_INBOUND)
        readfd = &fd;
 
    if(dir & LIBSSH2_SESSION_BLOCK_OUTBOUND)
        writefd = &fd;
 
    rc = select((int)(socket_fd + 1), readfd, writefd, NULL, &timeout);
 
    return rc;
}

Why waitsocket() is not called in every case? For example: for a successful handshake to happen, doesn't it make sense to wait for the socket to be readable and/or writeable?