The criteria used to design a vehicle are based in large part on the features that would sell to the widest audience at a reasonable price for the vehicle's class. Reliability, price, and fuel economy are typically factors in the decision-making process.
The aftermarket has become the means by which one may customize one's vehicle to suit a particular personality, to provide a role not satisfied by the stock vehicle, or to simply stand out. Aggressive styling and/or performance enhancements, which might not be important to the average buyer, usually make up a significant portion of the aftermarket. In fact, some parts come bundled together within a larger kit that one may choose to install in order to alter one's vehicle to a more significant degree.
The aftermarket also caters for the wish of many vehicle owners to have access to replacement parts that are less expensive than the official spare parts produced by vehicle manufacturers.