Your onset date is the date your disability began. The date represents a confluence of when you were not working at SGA and when you had a medically determinable impairment that is expected to last a year or more. You can read about the factors affecting the onset of disability in Social Security Ruling 83-20.
The "alleged" onset date is selected at the time of filing a disability claim. Once you receive a favorable decision, it just becomes your onset date. The onset date is important for Title II disability benefits, because it establishes the beginning point of your disability. Benefits are calculated with that date in mind.
The onset date that would maximize benefits for a disability claim is at least 17 months prior to your filing date. That allows for the 5-month waiting period and the 12 months of retroactive benefits available for disability claimants.
In an SSI claim, the filing date is treated as the onset date, since there are no SSI benefits payable prior to the date you file for benefits.