Here’s an app note from Maxim describing the use of a digital potentiometer in analog circuits like amplifiers and filters. By using a digital potentiometer in an op-amps feedback loop, a digitally controllable variable gain amplifier is constructed.
Variable gain amplifiers often use a mechanical potentiometer to set the gain. An example is a volume control dial. However, when the analog signal path is digitally controlled, a digital potentiometer might be used to control gain. This article discusses the use of the digital potentiometer to form digitally controlled gain or filter blocks.