Voice gateway or CUBE? Either way, they do more than just handle how calls are routed and modified. They also provide transcoding, conferencing resources, and media termination points for calls on hold coming from systems that may not support on hold calls. These are known as hardware resources that are put into the router PVDM2 or 3s. Additionally, for a CUBE, you can get increased security in who can traverse your IP to IP gateway with authentication. Finally, SRST and SRSV is available in the event your WAN/LAN link goes down to your CUCM so your phones can temporarily register to your voice gateway and use voicemail. There is much more but this question is so broad....If you don't understand the technology you might want to start with CCNA Voice.
