Mainly, for PROFIT!
In the mid '70s through the early '90s, when I was doing automotive aftermarket, car dealers would order a lot of stripped units, then have companies like I worked for "preload" the strippers with stereos, pin stripes, paint and fabric protection, and they would make a LOT more markup on the add-ons versus the profit they'd see from the same factory accessories.
Then there would be further "upsale" opportunities when the vehicle sold. Tape units or upgraded sound systems, CB radios, power antennas, cruise control, fog lights, sunroofs, air conditioning, and later cell phones.
It was kind of a racket. Buyers pretty much had to take the preloads, and low quality goods and half-ass installation caused complaints. The car manufacturers started cracking down on the practice, and the technology changed anyway so that the practice went away. I guess that I was in it in its true "glory days", and it was a fun way to make a living.
The dealership parts counter also sold accessories (as in the dealer installed AC), as the Parts Dept. was also a profit generator. But the aftermarket was cheaper, in the parts and especially in the non-dealership labor cost.
As Paul Harvey used to say, "Now you know the rest of the story".