When to use EnableViewState = "true" ?

Question

Dear experts,

 

I would like to know when to use EnableViewState = "true"..cos here are two lines of codes but only supplier is using and Category is not.

"ddlSupplier" runat="server" onSelectedIndexChanged = "ddlSupplier_SelectedIndexChanged" EnableViewState ="true" AutoPostBack = "True">

"ddlCategory" runat="server" OnSelectedIndexChanged="ddlCategory_SelectedIndexChanged" AutoPostBack="True">

tks.