For at least a few people, there is an initial reluctance to say they are “not guilty” at an arraignment. They are not interested in going to trial and have hired a lawyer just to mitigate the damage. They feel it is dishonest to say they are “not guilty” when they are guilty, but, legally speaking, when people say they are “not guilty,” they are not necessarily asserting their innocence. It is a little more nuanced than that. Instead, it means that someone is putting the government to their burden of proof and making them prove everything that a charge entails.
A plea of not guilty can later be amended to a guilty plea, but the reverse is not true. Once a guilty plea has been entered, there are limited situations where a guilty plea can be undone. Even if the ultimate goal is to take a plea offer, a defense lawyer needs something to negotiate. A potential guilty plea often is the best bargaining chip.
Incidentally, saying “not guilty” does not have to be limited to courtroom settings. Interactions with police officers could also be an occasion for saying, “not guilty.” Whether it stems from a simple traffic stop or a more serious allegation, the phrase is different than an outright denial. There are numerous laws, covering many situations, which make it illegal to lie to police officers. The technical meaning of the phrase leaves open the possibility that someone is not actually denying the allegation. Some people find this more comfortable to say than asserting the right to silence, which they believe makes it sound like they are hiding something. Regardless of which option people choose, it is a good idea to choose one and let the officer know that they heard the question. Also, the Supreme Court has said that people must affirmatively assert their right to silence if they have not been arrested yet.