I use one, but it doesn't make a huge range difference. You get a better signal and less drop offs, especially in big seas.
Wifi is line of sight so the higher the shore and boat antenna, the higher the range, in theory. But wifi doesn't go over or around mountains. I get my best range where the cell tower is on a high bluff. A usual trip for me is Astoria via inside passages to Alaska. I consistently get coverage at 25 nm if there isn't something big blocking the signal. When traveling, I use Verizon in the US and Canada and as long as I'm in the 25 nm range of a clear tower, I have wifi. Maximum range I achieved was about 35 nm.