You may not need to call a criminal defence lawyer for every minor citation you receive; a traffic ticket or other such infraction may not have much of a defence to begin with, and the cost of such a small ticket can be much less than the cost of a lawyer, no matter how much you resent having to pay that fine! However, there are times when it's good to call a defence lawyer to ensure your rights are protected and to ensure you get a fair trial and hearing, or to help avoid such a trial in the first place. Note when it's always good to call a defence lawyer in your case.
If a juvenile has been arrested
Juveniles can be charged with crimes and, depending on their age and circumstances of the offense, they may face the same type of citation and consequences as adults. However, a juvenile may have special means of defences that should be presented to the court; for example, if it can be demonstrated that they didn't understand the consequences of their actions or if they were unduly influenced by an adult. In all cases, it's best to meet with an attorney just to ensure you know the rights and defences of a juvenile in all criminal matters.
If you're being asked to interview with the police
The police may not have charged you with a crime yet, but the information you provide during an interview may be just what they need to file those charges; that interview may then also be part of their evidence against you in court. This is why it's good to have a defence attorney with you when you talk to the police even if you think you haven't committed a crime, as you don't want to give police too much information or express things in a way that can be misconstrued.
If you don't understand the evidence against you
The evidence that is presented in a criminal case can often be difficult for anyone to understand; forensic evidence may include DNA analysis, fingerprint analysis, information taken from your cell phone or computer, and so on. These things may be very difficult for you to comprehend, much less argue against.
A criminal defence attorney will usually be better prepared and more experienced in going through all that information and being able to understand its implication when it comes to charges against you. They may also have their own legal experts who can present evidence on your behalf, or who can refute evidence with their own scientific analysis.