You must use a ScriptManager control on a page to enable the following features of ASP.NET AJAX:

1. Client-script functionality of the Microsoft AJAX Library, and any custom script that you want to send to the browser.

protected void Button1_Click(object sender, EventArgs e)
{
    Page.ClientScript.RegisterStartupScript(
        this.GetType(),"myscript","alert('hello world!');");
}

2. Partial-page rendering, which enables regions on the page to be independently refreshed without a postback. The ASP.NET AJAX UpdatePanel, UpdateProgress, and Timer controls require a ScriptManager control to support partial-page rendering.

3. JavaScript proxy classes for Web services, which enable you to use client script to access Web services by exposing Web services as strongly typed objects.

[WebMethod]
public int Add(int a, int b) { return a + b; }

function CallAdd()
{
    // method will return immediately
    // processing done asynchronously
    WebService.Add(0,6, OnMethodSucceeded, OnMethodFailed);
}

4. JavaScript classes to access ASP.NET authentication and profile application services.

Sys.Services.AuthenticationService.login
Sys.Services.AuthenticationService.logout

<script type="text/javascript">
    function MyMethod(username, password)
    {
        Sys.Services.AuthenticationService.login(username,
            password,false,null,null,null,null,"User Context"); 
    }
</script>

See more at https://msdn.microsoft.com/en-us/library/system.web.ui.scriptmanager.aspx