If you have trained Jiu-Jitsu for more than a few months then you've probably competed or the idea has at least entered your mind. There are a variety of reasons why people choose to participate in competitions and I'd like to look at some of those motivations.
1. Exciting experience
The idea of going somewhere that is foreign (not your home gym), surrounded by strangers that are seemingly watching you, and going into battle against someone you know nothing about is exciting and sometimes scary feeling. To me there is something exciting about that and I don't think I'm alone in that thinking. This is hard to replicate in your own gym. I feel this is taking Jiu-Jitsu and the rolling experience to the next level.
2. Test yourself and techniques
If you are regularly rolling against the same guys at your gym it is very easy to learn each others games. There is nothing like rolling against someone you know nothing about. This is the time to play your "A" game. This is not the time to experiment with techniques you don't know well. Win or lose you are going to learn something about yourself that you won't necessarily learn training at your home gym. My coach regularly tells us that competing raises the level of your game and a path to improvement. I think there is a lot of truth to that because if you have holes in your game they will likely be exposed here and the things you do well will possibly shine.
3. Gain notoriety among peers
Let's be honest, most people have some desire to compete or at least taste a little glory. Some achieve that satisfaction by winning small battles at their home gym and others do it in competition. I don't know anyone that doesn't care what their peers think. How much stock you put into that is up to you but I think it is nice to have supportive teammates that can be happy for your competition experiences.
4. Some coaches promote based on competition
I know that coaches have different criteria for promoting students. Promotion through competition is probably one of the most difficult routes to take. Why is that you ask? Well it's probably safe to say that a coach may have a more strict criteria for promoting a student that competes a lot versus the BJJ hobbyist that trains twice per week and almost never competes. Students that compete are a representation of their gym. What coach wants a student that competes a lot to be rushed through promotions and is getting crushed in competition?
5. Some are doing this to make a living
Let's face it. The majority of us are hobbyist grapplers ranging from someone that wants to do something different to get in shape all the way to the very serious competitor regularly pushing themselves in competitions. There are a select few that are professional grapplers and do very little else but focus on training for a living. Some of us go to secondary schools to become proficient in a field for our careers; others are training Jiu-Jitsu with the intention of making a living from it. I have run into my fair share of guys in tournaments that fall into this category. They are generally the toughest, most technical people you will compete against.
Why do I compete?
I compete for a variety of reasons. I like the experiences I gain from competition and how it makes me grow as a grappler. I like the memories that are created (hopefully good ones!). I enjoy breaking down video and learning from my mistakes and looking at the things that I did right. I also enjoy the historical record that is created for myself that I can revisit in the future. I like representing my gym and the brand of Jiu-Jitsu that I have shaped into my game.
We all have our motivations for training Jiu-Jitsu. Participating in competitions isn't for everyone and there is nothing wrong with not doing it. That said I do feel that anyone that is serious about Jiu-Jitsu and plan to be in it for the long haul I can't stress how beneficial competing can be towards shaping your growth in the sport.
This highlight video from the 2012 IBJJF Worlds tournament last summer always turns up my motivational juices.