The net charge of an isolated system is conserved because total electric charge of an isolated system cannot change. This is because electric charge can be created or destroyed, so any charge that is created within an isolated system must be balanced by an equal and opposite charge that is also created within the system.

Another way to think about it is that the net charge of a system is a measure of the total number of positive charges minus the total number of negative charges in the system. If no charges are added to or removed from the system, then the net charge will not change.

This conservation law is important in many areas of physics and chemistry, such as electromagnetism and electrochemistry. For example, in an electrochemical cell, the net charge of the system is zero, even though there are different numbers of positive and negative charges in the different half-cells.