The specification is based on Thunderbolt and promises up to double the transfer speeds of existing USB specifications. We could see speeds leap from 20Gbps to 40Gbps in "certified" cables, according to the USB-IF. This is a tremendous improvement and one that could pave the way for an entirely new wave of high-speed devices.
As the USB-IF has noted in the past, 40Gbps speeds will allow users to do things like connect two 4K monitors at once, or run high-end external GPUs with ease, to name just a couple potential use cases.
Though the USB4 specification is now finalized, we still need to wait for device makers to create gadgets that will take advantage of it. Unfortunately, that process could take a while, and we wouldn't expect to see any USB4-powered consumer products (including monitors, thanks to USB4's hybrid data/display protocols) hit the market until at least early 2020.
It'll likely take even longer for mainstream motherboards, PC cases, and laptops to adopt the specification. Even now, many such devices still include only one or two USB 3.0 ports, let alone USB-C or 3.2 alternatives.