Speed is the primary benefit.
Theoretically, you can access the Internet at twice the speed of your old 28.8K modem. Though all v.90 modems are 56K modems, not all 56K modems are v.90 modems. A v.90 modem is a 56K modem based on international standards. 56K modems other than v.90 do not conform to the standard and the different technologies used by them are incompatible. While these other modem types could not communicate with each other at high speed, all v.90 modems will be able to.