Phono Pre-amp
As turntables fell out of favour with consumers many manufactures of amplifiers and receivers deleted the phono pre-amplifier stage from their electronics. If you are one of the growing band of people still using a turntable, and your amp does not have a phono stage, you will need an external device.
The other reason you may find to purchase an external phono stage is if the one that is incorporated into your amplifier is not of sufficient quality. This is not an uncommon situation, with many of the companies who still build this pre-amp stage into their amps opting to a rather basic and inexpensive option.