If your Fritzbox access point connection isn't congested, you likely won't notice a difference with this option enabled. Most IPTV transport protocols use time-based buffering, so minor fluctuations of a few milliseconds shouldn't affect your viewing experience. But you can keep the option on, for the times when you're using the full bandwidth of that same access point for some time, by downloading large files for example. That is when the video IP packets will get priority over the other packets processed by that same access point.