Have you noticed how many of the objects around us beep or buzz?
From a microwave to a phone on to a lift to a bus.
I believe there are two possible reasons for this: design in isolation; and featuritis.
Designed in isolation comes from not being aware of or not being concerned with the big picture of the end user's life. The designer assumes that the product he or she is designing is the only product that is worthy of the end user's attention.
Featuritis is simply an attempt by manufacturers to differentiate their products in the least expensive and most superficial way possible. Instead of fundamentally thinking about how to add real value to a user's life, and therefore making their products naturally differentiated.