According to the Elven E. Helms masonic lodge website:
The purpose of the chamber of reflection is to give the candidate a chance to prepare himself for his initiation into the Fraternity. It is not just for the external preparation of the candidate, to put on the required garments, but for the internal mental preparation of the candidate. The candidate is encouraged through self reflection to contemplate what his motives are for joining the lodge.
The chamber of reflection is exactly what it implies, a quiet room where the candidate is to meditate before his initiation. Isolation in this cavern-like room is where a symbolic metamorphosis is experienced, the neophyte emerges from this chamber symbolically transformed into a new person. It serves to separate the candidate of all earthly things, his family, his job, the superfluities of daily life, and makes him consider the notions of his own mortality.