You are technically correct that the Importance textbox could be set declaratively by doing this:
<asp:TextBox ID="txtImportance" runat="server" Text="0" MaxLength="256" Width="100%"></asp:TextBox>
However, that wouldn't work if we want this field to participate in the update, because there're no databinding expression.
If you actually try this, the ObjectDataSource will throw an InvalidOperationException:
ObjectDataSource 'objCurrCategory' could not find a non-generic method 'InsertCategory' that has parameters: Title, ImageUrl, Description.
It is the Bind expression that let's a value participate in the two-way dayabinding process. If you try to set the Importance field declaratively, without a Bind expression, it cannot participate as a parameter in the two-way databinding. Therefore, it never gets sent as a parameter to the InsertCategory method.