Let's say you're somewhere in Europe and you want to explore the continent for cheap. You're not sure when you'll have free time in the four months you'll be there, but you do know you'll want to travel for about five days in the Netherlands, Belgium, France and Germany.
Unspecified as your plans might be, that is not a problem! Eurail is perfect for vague plans. What you can do is buy a pass that allows you unlimited travel during a set amount of days within a set timeframe inside the countries of your choice. Could be a good deal!
Or if you're in New Zealand, you should know that flying is often cheaper than driving or taking the train. You should also know that there are loads of bus companies, and there are at least three kinds of bus passes.
If you're in Brussels, you'll use the metro often. You could get a season pass right away and save yourself a lot of money.
... and the list goes on. The point is – do your research on transportation at your destination. It could save you money, and you'll probably come away from your reading with new-found respect for public transportation.