Conductivity is important in a number of industries.
It is particularly important in hydroponics. Pure water does not conduct electricity. Nutrient solutions are full of impurities due to the electrically charged nutrient ions. Measuring conductivity lets you manage the strength of your nutrient solution. A weak solution will mean that the plants lack key elements to grow. A low measurement means that you need to add nutrients to your solution. A high reading can lead to root burn. You will need to add water to dilute the nutrient solution. Your conductivity meter lets you manage using data rather than guess work.
EC is also monitored in aquariums as an indicator of changes that may need additional tests to identify. For example, an increase in nitrates will show as an increase in EC. More broadly, EC is used for environmental monitoring and in industrial processes.
The main advantage of conductivity measurement is that it is simple and fast, making them useful for making routine assessments. This reduces the need for more expensive specific tests.