Right now, space travel is all about carefully moving between orbits. If you want to go from Earth to Mars, you wait until the two planets are correctly aligned, and then place yourself into an elliptical orbit around the sun so that the apoasis of the orbit hits Mars. You only have to make a single, long burn at the start of the orbital change, and if you do it right you'll fall into a neat Martian orbit.
Using this method, it takes quite a long time to get to Mars! It seems that most of the work getting there is done by gravity - the craft's engines only do a little.
It's as if the craft is a transistor - its engines provide a little seed current so that gravity can do the rest.
I suppose we do it like this because fuel is hard to get into space, and our engines are not very good. 100 years from now, when these are no longer issues, how will spacecraft move from one planet to another? Will they still think about orbital changes, or will they just point towards the target and hit the accelerator?