Why do I need a phono-pre???

Depends on your cartridge and receiver/pre-amp. You may not. What do you have?

All depends on whether your receiver has a phono-stage in it. In the days before CDs, they all did; now most don't.

All that plus many of the cartridges today have a lower output voltage than many of the popular ones from an earlier era. I'm not sure that some of the lower output moving coil cartridges would be adequately amplified with the built-in phono stages of a mass market receiver.

Besides, audiophiles love separates! ;-)