This is something which u can install later, depending on owner own taste. May be for me sound quality of OEM setup is fine. But for certain people, it's not good enough.
Certain features, like VSC/VSA, Abs, airbags, meter cluster, transmission or other features like climate control, hill Assit, brake hold, push start, smart entry, auto retractable mirrors etc can't be upgraded later.
Also cost impact of a nice sound system is much less than that of other features.