what is the use of Buiness Logic Layer. why do not we call directly DAL from Presentation Layer?

See more:

C#

ASP.NET

Business entity layer.
***************
public class BEL
{
#region variable

private string firstname;

#endregion

public string Firstname
{
get { return firstname; }
set { firstname = value; }
}
}

presentation layer:
**************

protected void save_click(object sender, EventArgs e)
{
string output = string.Empty;

BEL ObjUserBEL = new BEL();
ObjUserBEL.Firstname = txtfirstname.Text.ToString().Trim();
BLL ObjUserBLL = new BLL();
output = ObjUserBLL.insertuserdetails(ObjUserBEL);

}
and i have one more layer DAL.cs (Data Access Layer).

here i got doubt that.
Instead of giving like below.
BLL ObjUserBLL = new BLL();
output = ObjUserBLL.insertuserdetails(ObjUserBEL);

i can give directly to DAL.cs layer right? like below

DAL objUserDAL = new DAL();
output=objUserDAL.insertuserdetails(ObjUserBEL);

when i browse, i got information like it is for validation. i do not understand and not find any validation.

so, what is the need of BAL (Business access Layer) ? without BAL also, it calls DAL layer and do all function.
kindly clarify me..