One of the most important things that we tend to forget as martial artists is the importance of mixing things up every once in a while (hence, variety). For many reasons, such as tradition, simply practice, repeating what we're taught, or simply because we many not know any better, it's very typical to settle into a routine and not vary from it. Of course, there are things that need to be repeated over and over again in order to commit them to muscle memory so they become second nature. For example, it's very likely that karate people have done millions of reverse punches, right?
While some repetitions is important (and some times unavoidable), there is are a few problems with sticking to the same routine day after day after day. For one, from a physical standpoint, the genius of the human body is its ability to adapt to any given circumstance. Muscles and bones adapt to weight training by getting bigger and stronger; the cardiovascular system responds to aerobic training by moving oxygen more efficiently to shorten recovery times; muscles release less and less lactic acid as they get accustomed to stress (in layman's terms, your muscles are less and less sore as you get in better shape); the list goes on. Do you remember performing a particular workout (or a single exercise) to the point where it doesn't seem to give you the same benefit that it did when you first started it? That's your body adapting.
Another good reason for variety is so simple that it's easy to miss it when it's obvious: it helps eliminate boredom. How many times in your life have you stopped doing something because it simply got boring doing the same thing over and over again? Outside of the martial arts realm, we see this all the time with couples in relationships that are trying to "add some spice" to their relationship. The same thing applies here as well.
How do you change things up? That's up to you. It could be as simple as changing the order in which you do things in your workout or class. Instead of working on basics first, try doing kata work first and leaving basics for last. Try doing a class in street clothes instead of a gi. Perform the bench press with dumbbells instead of a barbell. Switch up the time of day you work out for a couple of weeks. Do a class or two outside. Have a class in the dark (with enough light for safety reasons of course!). Run a fartlek workout instead of jogging 4-5 miles at the same pace. Perform katas as fast as you safely can without worrying about technique. Do whatever in the world you can think of!