Graphs can be very useful to monitor functions of the body like heart rate, blood sugar levels, cholesterol levels, temperature. If you have asthma for example, you might need to graph your peak expiratory flow every day on a chart to monitor your breathing. If the graph looked like it were taking a downward turn, you would tell your doctor.
Graphs are also very useful when it comes to money. If you graph your spending over a year you can see periods of when you were spending a lot and try and work out why. If you understand more about your spending, you might be able to reduce it.