"Cleaning the signal" is not something you're going to find in a textbook on electronic technology. They are likely used as general circuit components throughout the design.
Ceramic capacitors are low cost capacitors. When compared to the ideal capacitor, they are inferior at audio frequencies because they change capacitance with Voltage and temperature. Film type capacitors perform better in this area. Ceramic's good points are low cost, small size and very low impedance at high frequencies.
All these characteristics are taken into account by the Electronic Engineer who is designing the product and must weigh cost against performance of the product. Often, it is the Bean Counters in the back room that exert an overriding influence on design decisions.
Ceramic capacitors are low cost capacitors. When compared to the ideal capacitor, they are inferior at audio frequencies because they change capacitance with Voltage and temperature. Film type capacitors perform better in this area. Ceramic's good points are low cost, small size and very low impedance at high frequencies.
All these characteristics are taken into account by the Electronic Engineer who is designing the product and must weigh cost against performance of the product. Often, it is the Bean Counters in the back room that exert an overriding influence on design decisions.