STAR's depend on a number of factors, including runway in use and traffic load. Often you'll be put on a STAR to ease the coordination between ACC and APP, and give APP a nice flow of aircrafts following specified paths, seperating them from aircrafts following SID's.
At some point they're taken og the STAR's for vectoring for a nice spacing on the final.
So the STAR's and SID's are there to ease the load on controllers, making them able to handle more traffic.
You'll often see more direct routings for a fix on/or close to the final during hours with less trafficload, or more direct tracks after departure.
So filing a STAR in your flight plan, won't really do any difference to the STAR given (pre-coordinated between ACC and APP/sector and sector).