There are many reasons why you need a lawyer after being arrested for a DUI...
It is highly advisable that you retain legal counsel before you just plead guilty to a DUI thinking that it is your first offense and what is the worst that can happen. You may be surprised how many things a good attorney can do to either get someone out of a DUI or make the probation, fines, or jail time less. And even more importantly to some clients, there are some good options available to help try and keep a person's driver's license. There are some people that believe they can save the legal fees by trying to do it themselves only to find out that they ended up paying far more in the long run in classes, high risk insurance, and lost days from work, than if they would have hired a lawyer in the first place. Not to mention the great risk they put themselves in by standing in front of the judge and hoping for the best.
DUI law is a highly specialized area of the law and many people not only don't understand the "rules" about how this type of case or guilty plea might affect your driving record, but also how it might affect your insurance rates, your job, your ability to drive, to get student loans, your ability to stay in this country, your ability to work in a bar, restaurant, or in certain professions like nursing, adopting kids in the future, or a myriad of other things.
We have clients come to our office everyday trying to "fix" mistakes they made when they simply plead guilty, did a diversion, or worked out some kind of "deal" on their own with a court or prosecutor. A prosecutor is not your friend; he is not looking out for your best interest. There are some things we can actually "fix" after the fact, while others, we have to tell our clients they are out of luck.
There is a reason that a lawyer is generally able to work something out with a prosecutor in many situations (plea bargain) and that is the threat of litigation. You are probably not a threat to a prosecutor on your own, but if you hire a lawyer who potentially will not only make that prosecutor work to win his case but also make it a real possibility that he could actually lose his case, then a prosecutor is far more likely to either dismiss your case (if he thinks he is not likely to win anyway) or work out a plea bargain.