A gigabit switch is handy for allowing devices that are all inside the house to fling bits at each other at a gigabit per second rate. So if you're streaming a video from your home server to your HDTV, a gigabit switch will not slow the stream down as it passes through from the server to the TV.
A video stream that originates from outside the home, however, has to go through your ISP, and usually that shows up in your house as either a DSL or a cable internet connection. DSL connections typically are no faster than 16 megabits per second into the home, and cable internet is usually no faster than 50 megabits per second. In the US, usually much, much slower.
Even optical fiber connections are typically limited to under 100 megabits per second. However, a gigabit switch does not slow the stream down once it gets there. Older switches (and hubs) provide 100, 10, and even 3 megabits per second.