Usually , nics onboard are controlled by the CPU.
What this card has is its own NPU (Network processing unit) or basicly, it can take care of things on it own, releaving the load from the CPU with Network-tasks.
Now, this will only work tho, if you play a game thats already maxing out, is having 100% load all the time and has trouble showing those high FPS.
By installing this card, it basicly means that you will lower the CPU time from its networkingtasks, so the CPU has a little more time pushing out those FPS.
But, this will be only noticable on lower end systems for example. And there's no game that will really use your 100Mbit NIC at all, many of home connections are'nt even designed for extreme hard uploads.
This card will give like 0.19999% boost, but basicly its not worth the money, and overal this product is hyped up. Even tho it changes the way it sends out packages towards your ISP, your ISP cannot send packages back faster then it already does.
But if you like the status of having a NIC killer card in your PC, be my guest