With that in mind, I hope the following comments explain why something does or doesn't work:
$("#Button1").get(0).value = "Click Me"; // it works; single object, no matched set;
// this object just happens to have a value property.
$("#Button1").value = "Click Me"; // it doesn't work because the matched set
// doesn't have a value property or method so it doesn't
// know what to with it.
$("#Button1").get(0).addClass("NewClassName"); // it doesn't work because addClass is
// a jQuery method defined for the matched set,
$("#Button1").addClass("NewClassName"); // it works, because the matched set has an
// addClass method. It loops over all elements in the
// collection (in this case there should be only one) and
// applies the CSS class you specify.
Hope this helps,