Yes, boxing can help you in a street fight. But never forget, it's more about an individuals mindset then anything else. Some people are more inclined to 'react' in a real life situation then others. Real life situations happen so fast, they can catch you off guard. And don't forget, there are no rules on the street. Real life situations are not a sparring session, you can get seriously hurt, specially when weapons are involved. Boxing will give you the confidence and sharpness of mind to react much more quicker then if you don't do anything which involves combat training, or you just sit around chilling and smoking all day.
Boxing or keeping fit in general will keep you more alert, but in order to get the full benefits from boxing you have to spar regularly. It can be a scary feeling when you are going to spar for the first time (I've been there myself), but believe me, once you do it a few times you start to love it. Just be careful, it can matter hugely on your first few sparring experiences, such as, if you go boxing and want to spar but you end up sparring a more experienced boxer, and they want to look good in the gym by giving you a boxing lesson this is not good and could totally put you off boxing and sparring forever. What you need for the first few times is good guys to spar with, this is one of the most important things.
Sparring is about learning, not taking someones head off. You might spar, get beat up badly and never go back boxing again, but you have to be aware, getting good at boxing sparring takes time to perfect. When I first went boxing I was going into sparring and treating it like a real street fight, just going crazy swinging everywhere! But after quite a lot of sparring I started to slow down, become more controlled, and really started to learn how to box.
The guys I sparred with when I first started seemed pretty good at the time, but once I got the hang of sparring they seemed like novices. You have to remember, everything takes time to perfect, from driving to boxing. And having good guys to spar with is one of the best things you can have, guys that understand that it takes time and want you to learn, who have nothing to prove because they know there good anyway, they don't need to show off by beating a novice. When you find training partners like that hang on to them.
Boxing is always handy in a street fight for many reasons-one being strength: Boxing training is tough, so this alone will make you stronger and more efficient with movement and co ordination.
Your Reaction time will be much quicker and after a while, boxing movements and punch combinations will get so drummed into you, it will become part of your natural instinct. Boxing training will also help you get faster on your feet, so you could jump back or duck out of the way of a street attack quicker. Your Confidence level from boxing training and sparring will hit the roof, specially when you know your getting better.
In general, boxing will just make you feel better all round, and when you feel better physically and mentally this will help you in any situation, from stress at work, getting the girl of your dreams, and yes even being being able to defend yourself in a street fight, which is only a good thing. No matter what walk of life you are from, or where in the world you come from, at some point in your life you will need to defend yourself. So doing boxing, or any other form of realistic fight training will definitely benefit you when the going gets tough.
A word of warning: Boxing can help you in a street fight but nothing is certain. There are a million and one different "real life situations" you could find yourself in, some which all the boxing skill in the world won't be able to get you out of. Street fights are not fair, mostly quick and brutal, anything can happen. Boxing has rules and regulations - the street doesn't, and some of the people you will fight there won't, and will do almost anything, or stoop very low to win or achieve their objective. So it's very possible that you might have to show some heart to overcome the situation. Never forget that, and never limit yourself to one fighting style. What if a fight turns into a wrestling match and ends up on the ground? Also remember: Your will to win can be just as, if not more important then your technique. You could be caught by the element of surprise, something that can catch anyone off guard so be aware.
If you are interested in what really works in a street fight and how to prepare for one I highly recommend you read the book below. If i could recommend one book you should read regarding street fights and what really works in real life situations this would be it. I cannot recommend it highly enough. You have to read this book.