Your internet service provider probably claims that your connection can deliver speeds of 15 and 20 megabits per second (that's pretty fast). But that Wi-FI router you have likely claims it can transfer data at a gigabit per second (that's absurdly fast).
The fact is you'll never see gigabit speeds -- ever (not even if you're lucky enough to have Google's ultra-fast Fiber network). Most of the Internet just can't deliver you anything close to that.
So why advertise those crazy speeds? The faster your Wi-Fi router, the bigger the pipeline for all those videos, songs and games that a growing number of people are streaming on multiple devices simultaneously. That means more people can be connected at once -- even if that connection isn't as fast as you had hoped.