You're probably aware that a weekly halfhearted carpet-vacuuming job, although it makes things look spic-and-span temporarily, isn't a thorough cleaning. You suck up the surface dirt but leave most of it lurking close to the ground. When you realize it's time for a deeper cleaning (which should be about once a year), you might be tempted to rent a steam cleaner to do the job yourself. It'd be cheaper, sure, but it'd take forever, and the results wouldn't come close to the work of carpet-cleaning professionals. They have the heavy-duty equipment, so let them come to you.