Web Forms :: Validator Control - Using A Server Script Instead Of Using Client Script?

I was trying to check how far validator works, if we disable javascript in client browser.

I've disabled scripting in my browser. Below is my aspx page

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
ControlToValidate="TextBox1" Display="Dynamic"[code]....

Surprisingly on the clicking button5, the page is getting posted bak, even if TextBox1 doesn't have value. I even found out , if we make the code change as this

protected void Button5_Click(object sender, EventArgs e)
{
if(this.Page.IsValid == true)
Response.Write(TextBox1.Text.ToString()+"Added Text<BR>");
}

Its not posting back..

if the server validators depends this much on client script, what's use of using a server script instead of using client script.