With the proper grounding outside, do I need the extra protection?
Tough call. Part of the answer is what's your individual paranoia level? Part of the answer is where do you live? If its in an area of the US with a large amount of cloud-to-ground lightning strikes, your odds of getting hit increase.
The unit you linked to appears to be a surge protector based on a gas discharge tube. When a transient voltage occurs that exceeds the breakdown voltage of the gas (generally 90 volts), the gas will ionize and become conducting. No longer is the impedance thousands of megaohms, but is now only a few milliohms. In other words, this transient sees what is virtually a short circuit towards ground and it is this shunting of the surge to ground that saves your ass.
You'll find a few variants on that link. Some will look like this:
Here we have a unit similar to yours but with the exception that there is a grounding terminal. Instead of installing such a unit next to your equipment, it would be placed in close proximity to your grounding rod outside your house and you would run a straight (no kinks) ground wire (I don't recall the gauge...it's thick though) to the grounding rod. Locating the unit there provides superior surge protection because of the close proximity to earth ground. Don't get earth ground confused to say the ground wire on your receptacle. There's a world of difference once a surge comes in.
You'll find such units specified with parameters such as...
insertion loss (usually a fraction of a dB...smaller is better as there's less signal attenuation)
breakdown voltage
the bandwidth that it covers (it has to pass the signals that your coax is handling...usually it'll be something like DC to 2 or 2.5 or 3.0 GHz)
Since it appears you have your own house, you may wish to consider a whole house surge protection scheme. Search under my name and those terms and it should give you enough to put you to sleep.
