There are several main reasons to have a migraine diary. It could help your doctor to make diagnosis, it helps you to identify your triggers and manage medications.

Diagnosis

Migraine is a complicated illness and its symptoms are very varied from one person to the next. There are no tests for migraine, it can’t be seen on MRI or other diagnostic devices. That’s why migraine diary is a key for your doctor for making a diagnosis. Your history will help you and your doctor to monitor and treat your condition.

Trigger management

Identifying your personal triggers is an essential part of reducing and preventing migraine attacks. Migraine diary helps to recognise triggers or warning signs and show any patterns to attacks. This is one of the most significant ongoing roles of a headache diary.

Medication management

Migraine diary can help you to assess whether your acute or preventive medication is working.\nAlso you need to track how often you are taking medication. If acute medication used too often they can cause medication overuse headache.
References:
  1. "Managing your Migraine.", Dr. Katy Munro. 2021
  2. "Headache Diaries & Migraine Diaries", Headache Australia
  3. "Why keep a headache diary?", The Migraine Trust