There are many disease in the world and all of these illnesses impact different people in different ways. But still, the symptoms broadly remains same. Thye help us in identifying the condition and also do the diagnosis.
A lot of times we go to the doctor and say, we have headache. He asks us since when you are having headache, where it is impacting you, how severe it is, etc. etc. Based on these questions and the answers which you give, he can recommend a opinion or judgement. If he is doubtful, he can ask you take a test.
This is the way, people diagnose illness and hence, take the first step towards its treatment.