After some time you find out that your car is using a lot of fuel, more than usual. An increase in consumption can be related to several factors. Often the main cause is lack of maintenance. Think, for example, of dirty or clogged filters, a dirty fuel system, dirty injectors and even a tire pressure that is too low can lead to higher fuel consumption.
A fuel additive can reduce fuel consumption if, for example, there is internal contamination (injectors or fuel system). Such an additive is also called a fuel system cleaner and you simply add it before refueling.