I know quite a few people who use bikes as their primary transportation and don't own cars. For them, a bike is for visiting friends, hauling groceries, taking young children to day care or school, and many other purposes.
Some people I know choose bikes for other reasons. If someone has a medical condition that makes it unsafe for them to drive (such as epilepsy or poor vision), they may ride because driving is not an option. Riding a bike gives them freedom and allows them to get around without having to depend on someone else to give them a ride.
A growing number of seniors use utility trikes as an alternative to driving - for grocery shopping, going to socialize with friends, to visit the library, etc.
Our bike infrastructure and traffic laws should accommodate all types of riders, regardless of what they ride and why they ride.