Every athlete competing in their chosen sport, should at some point in their training seek to develop their strength. Although you may not always associate being strong with having good cardio, strength training isn't limited to only improving your overall strength, but has a dramatic effect on your cardiovascular conditioning also.
When performing any movement, transition or submission during a BJJ contest, you are using a certain amount of strength. Technique is always the most important variable when it comes to succeeding in Jiu Jitsu, but no matter how how efficient you are with your energy, you will still need to rely on some element of strength.
When attempting, for example, a particular guard recovery in Jiu Jitsu, you will be using a certain percentage of your maximum strength in this movement (we’ll say for the moment that it’s 75% of your current strength capacity). After repeating this movement a series of times, along with other muscularly demanding movements, you will begin to fatigue. With regular strength training, you're able to increase your maximum strength capacity, and therefore use a lower percentage of your maximum strength to perform the same action. If we can bring that number (75%) down to say 50%, you'll be able to perform the same action quite a few more times before ultimately fatiguing.
This same principle applies to running, cycling, swimming, boxing and just about every other sport you can think of. So being strong isn't the only reason you should train for strength (although it is a good one).