Say you want to start a site about foreign chopper style motorcycles to help beginner learn about them and make their first purchase. You name the site 2WheelMotorMadnessDotCom because it gives a good idea as to what information lies within.
You start building the site and it contains info on Yamaha, Suzuki's, Triumphs, etc. But because the site's name (2wheelmotormadness) can also be used to describe domestic choppers, you add those to the site as well.
At this point, would you add further information like info on Touring motorcycles, sportsbikes, maybe even dirtbikes, since they all also ride on 2 wheels? Why or why not?
(Oh, and I just made that website name up. It's not mine, don't know if it even exists)