I think that there are a couple of reasons that Tom speaks to the audience. On one level, it is almost as if Tom is pleading with the audience for absolution in terms of abandoning the family. Tom seeks the adventure and the excitement that he perceives is not a part of his current life. Essentially, he seeks a life that is different from the one he leads. Understanding that there is a level of abandonment involved in this pursuit, Tom is quite content with being "a bastard son of a bastard" and leaving the family to discover this "excitement" that lives in a world outside of his own. Yet, there is some level of guilt involved with such a move. His speaking to the audience is a hopeful way of assuaging this guilt, trying to explain it away and breaking the wall between character and audience is a way to achieve this. Williams is a writer who explores the level of cruelty that family members display to one another. We see this in other plays, such as sisters betraying others' trust, or sons and fathers not being able to connect. In Tom's case, he is going to leave his family and in abandoning them, he speaks to the audience as a way to explain what he is going to do. For those who break these bonds, Williams might be asserting that there is some external force to whom they plead in order for understanding behind their actions. It is in this vein that Tom speaks to the audience.