Welcome to the forum and Trainz. Of course you are free to build in any scale/gauge you wish, but just think of it. You can create a route that has actual MILES between stations, not scale miles. If you happen to run across a route schematic that's measured in miles, then you can create it actually in miles.
It's kind of hard to get your head around. I know. I started out in HO and graduated to N and then found Trainz. I asked the same exact question you did. Once I realized I could build a 50 mile long stretch of track and have it actually BE 50 miles, I was happy as a clam.
Bill