This is not just a marketing ploy. For the best performance out of the balls you play with, you should match the ball type to the surface you plan on playing with them on. If you don't play tennis too often, you probably have some old balls in the closet that you grab before heading out to the courts. This might be fine for the occasional practice. If you play matches or if you are practicing for tournament or league play, however, you should open a new can of balls every time you play.
Tennis balls start losing their compression as soon as the can is opened.
If you have a strong grip, take an old ball and a new ball and squeeze them both. The difference is striking. Soft balls are useful if you practice alone against a wall or backboard. They don't come back as fast, making the experience more like full court tennis. You should use a regular duty ball on clay or indoor courts.