我不明白我将同一个变量赋值两次背后的逻辑

我第一次在这个网站上提问。我在Visual c# 2008中创建了一个简单的窗体，我将两个文本框的值添加到access 2010数据库中的两个字段。该数据库名为TestDatabase。accdb，表为TestTable。两个变量，FirstName &地址分配给文本框。txt的文本值。名字和文本地址。在我看来，我也在为OleDBCommand类添加参数的语句中为这两个变量添加值:

myCommand.Parameters.Add("@FirstName", OleDbType.VarChar).Value = txtName.Text;

所以基本上它是工作的，但我不明白其中的逻辑因为它看起来好像我不需要使用

FirstName = txtName.Text;

或

Address = txtAddress.Text;

语句。如果我从TextChanged事件中删除代码，我将收到几个警告。

代码如下:

namespace Test
{
    public partial class Form1 : Form
    {
        private string FirstName;
        private  string Address;
        private void cmdAdd_Click(object sender, EventArgs e)
        {
            string strSQL = "INSERT INTO TestTable(Name1, Address) VALUES(@FirstName, '@Address')";
            // represents an open connection to a data source.  Is a class
            OleDbConnection myConnection = new OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:''TEMP''TestDatabase.accdb");
            // represents an SQL statement or stored procedure to execute against a data source
            //( takes care of passing queries to the database).  Is a class.
            OleDbCommand myCommand = new OleDbCommand(strSQL, myConnection);
            myCommand.Parameters.Add("@FirstName", OleDbType.VarChar).Value = txtName.Text;
            myCommand.Parameters.Add("@Address", OleDbType.VarChar).Value = txtAddress.Text;
            try
            {
                myConnection.Open();
                myCommand.ExecuteNonQuery();
            }
            catch (Exception)
            {
                MessageBox.Show("Something went wrong");
            }
            finally
            {
                myConnection.Close();
            }
        }
        private void txtName_TextChanged(object sender, EventArgs e)
        {
            //IsNullOrEmpty indicates whether the string is null ot an Empty string
            //true if the value parameter is null or an empty string(""); otherwise, false
            if (string.IsNullOrEmpty(txtName.Text))
            {
                //has no value
            }
            else
            {
                FirstName = txtName.Text;
            }
        }
        private void txtAddress_TextChanged(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(txtAddress.Text))
            {
                //has no value
            }
            else
            {
                Address = txtAddress.Text;
            }
        }
    }
}