"Why should someone hire a lawyer for a arraingment?" [sic]
The court will want counsel or a signed waiver of counsel at arraignment. If you are claiming indigency, you will need to have that form completed and approved before the court will appoint counsel. The police reports and other evidence will also be provided to you (if pro se) or your counsel at that time.
If you show up without an attorney and do not proceed pro se, the court may continue the matter until you do produce an attorney.
The foregoing is for general information purposes and does not establish an attorney-client relationship.
The answer to your first question depends a great deal on the type of criminal case we're talking about. In some minor cases, the arraignment may very well be short and simple and the attorney may truly be doing little more than you yourself could do.
In more serious cases, however, arraignments will often involve a bail hearing, where the judge is asked by the prosecutor to impose monetary bail on you before you are released. In even more serious cases, there may be a question of pre-trial detention. Both of these are hearings that should only be handled by an experienced attorney. In fact, even if you normally would not qualify for a court-appointed lawyer, if there is a question of bail or detention at arraignment and you are representing yourself, the judge will likely appoint counsel, free of charge, for the purposes of bail only. That's how important it is for you to have a lawyer for this type of situation.
Keep in mind that when you hire a criminal defense attorney, you are almost always hiring that attorney to represent you for an entire criminal case, not just the arraignment. The arraignment will be included in your representation. Most criminal attorneys in Massachusetts work on a flat fee basis, meaning that you wouldn't necessarily save any money by waiting until after the arraignment to hire someone. It's also important to remember that the best attorneys will be investigating and working on your case even before arraignment if you hire them beforehand, meaning that they can be doing work out of court that is relevant and helpful to your case. After the arraignment, the case becomes more complicated and will involve law and procedure that only a trained attorney should handle.
To answer your second question, yes, you absolutely can move your pre-trial hearing. You will have to appear in court and file a motion to advance and continue the case. A judge will hear it, and the district attorney's office will have the opportunity to agree or object to the continuance (in this case they will usually agree). If you are represented by a lawyer at this point, the lawyer will be able to do this for you, often with little to no effort on your part.
You should do everything you can to avoid getting arraigned. If you can stop the arraignment and the entry of charges against you then it will look much better on your record.
"Not guilty" will still scare off a lot of people. "Not probable cause to arraign," however, is as close as most folks get to a finding of innocence.
You only have one shot at it. Use an attorney.
Do you want accurate, personalized, legal advice that you can rely on? You will have to hire an attorney, not ask on Avvo. I am not your attorney and am not creating an attorney-client relationship by this post. I am therefore giving only general advice. This advice may not apply to you or your situation; may not take account of all possibilities, and may not match the advice I would give to a client. DO NOT rely on this advice or any other advice on Avvo to make your legal decisions. If you want an answer to a legal question you should retain an attorney who is licensed in your state.
You present a gross oversimplification of the arraignment process. The arraignment can be an opportunity to avoid being arraigned altogether, thereby keeping the charges off your criminal record. Even if you cannot avoid being arraigned, the prosecutor may be seeking bail, and a lawyer is very helpful in arguing for a release on personal recognizance or a law cash bail. If identification is an issue, there are issues that must be handled at the arraignment to prevent prejudicing the defense in the future. If the proseuctors are asking for some conditions of release that you don't want to or cannot comply with, a lawyer is very helpful. These are just some of the many ways that a lawyer can help at an arraignment.
The fees charged to represent a defendant, while perhaps broken up into installments, are for representation of the defendant at all future stages of the criminal process, not just the arraignment. If you are arraigned without a lawyer and given a pretrial date, the court will accommodate a request to change that pretrial date when a lawyer has been retained.