While thermal curtains are ideal for many situations, especially for protection from extremes of heat, ordinary curtain have some of the same effects to a lesser degree, for a lesser cost. This can be a cost-effective option where the heating and cooling needs are very modest, or where the benefits of thermal curtains are minimal (e.g. in a hot climate where the sun does not directly hit a window).
Having thin curtains under the regular curtains has the advantage of allowing differing levels of lighting.