Allow me to get on my soapbox for a minute. The band-assisted pull up is a great exercise for those wanting to perform their first pull up proper. However, recently it has come under attack because it fails to help at the top of the pull up where help is most needed. I disagree. By using the band we are ensuring that we're not wasting our energy on the part of the movement that's easiest. This allows us to have the energy to tackle the hardest part of the pull up at the top. I'm not saying that the assisted pull up can't be abused. If you're using the band to build momentum while bouncing around then you're obviously not using the band in a productive way. But despite whatever potential drawbacks the assisted pull up may have, it still works well in helping us get our first pull up. As you can see in the demonstration below, the key is to allow the exercise to go through its full range of motion.
In short, pull ups -- including band assisted pull ups -- are a great body weight exercise to develop your upper body strength, specifically your back muscles. You'll also be working on your grip strength and your forearms because you're going to be hanging your body weight from the bar. This can be really difficult to do if you're just starting out, so it only makes sense to give yourself some assistance so that you can gradually work your way up to what your body weight is. Your ultimate goal is to work your way down to a small band and eventually no band at all.
Have a fantastic day!