Normalization of earnings is done for prior years to understand the cash flows generated by the core business in a normal year. Every year brings surprises. If these surprises are random one-off events that are not likely to occur, it is considered abnormal and removed to reflect what the earnings may have been in a normal year. Normalization is done to help you estimate future operating cash flows to use in a DCF valuation.
Analysts also normalize earnings to compare firms. This is done to be able to compare apples to apples and oranges to oranges.