VoIP gateways helps to split the workload. There is a large number of brands and models to choose from. Typically you can choose from features such as FXS/FXO interfaces, Ethernet, T.38 support for faxing, 911 support, PSTN fallback, T1/E1 interfaces, multiple codec support, jitter buffer and echo cancellation, QoS and packet filtering, conferencing, and so forth. They are good, cost-effective devices that solve a number of problems, with SIP NAT traversal and QoS at the top of the list.

Using a standalone VoIP gateway removes a significant processing burden from your iPBX server. Asterisk was invented back when x86 PC hardware was considerably less expensive than telephony gear, so it was cost-effective to use an Asterisk box as a PSTN gateway via Digium's own FXS/FXO interfaces, or those of competitors such as Sangoma and Rhino. But that cost advantage has disappeared, so your iPBX box can be just an iPBX, and not do double-duty as your PSTN interface.

Ideally you will have a dedicated Internet line for your VoIP. Putting a VoIP gateway between your VoIP server and the Internet, on its own dedicated Internet connection, makes it a lot easier to administer and secure, and you'll get better call quality. You'll also avoid conflicts with DHCP and DNS-VoIP servers that support auto-provisioning phones include their own DHCP/DNS servers, so trying to manage these in harmony with your existing nameservers gets complex. It's easier to dodge the problem entirely by putting your IP phones and servers on a separate network.