To add to Ed's comments,
If your traveling near Class Bravo airspace, you can generally expect to be routed around - rather than through - the airspace. There are exceptions, but ATC in many of those areas has the attitude "not in our airspace". That's true even where there are V-routes into the airspace. Houston, for example, will let you go over at 17,000 on the Victor airway that goes east-west... below that, you go to Scholes. Dallas, you need to be in the flight levels to cross the Class B unless you're landing in or under the airspace, in which case you go through it. Some will even keep you outside the airspace and send you on 50-mile detours if you're landing in or under the Class B. Just expect it.
In other cases, routing also can be dictated by flow-control.
Coming out of the West side of the Washington, DC area, I know I can get direct routings beyond a certain point, so that's what I file. Sometimes I get a full-airway clearance from Potomac, yet as soon as I'm handed off, the routing is changed to what I filed.
What scares me are the times I am handed off to another facility, and that facility asks me to give them both my filed and assigned routing because they never received the info from the prior controller. That happened to me on the way back from Missouri this last trip.
As Ed points out, if you fly the route often enough, you'll learn the route to anticipate.