Why don't we collect year of birth?
We collect month and date of birth so we can send them birthday rewards every year. We don't collect year of birth because the full date of birth including year is a sensitive personal information and most customers may not be comfortable sharing it. There is no reason for us to collect the full date of birth.
However, we do collect the age group such as 21+, 30+ etc and majority of customers are comfortable sharing this information. Age group is important for you to understand your customer demographics so you can tune your business to match your customers.