The origins of the sin sot are hidden in the past, although the concept of the 'bride price' is not new, even to the west. It was a form of guarantee that the bride was a virgin - the groom paid the family to ensure that this was the case. Not now, of course, but it is still a means of the bride saving 'face'.
If the woman was cheated by the man, the sin sot was to offer her cash in recompense. By paying the sin sot, the groom is accepting her as a virgin (even though she may likely not be.) In Thailand, even today, men still do not like the idea of marrying a woman who has been with another man, particularly a divorcee or a mother. If it is proved that she has been unfaithful, it is tradition that the sin sot is returned to him.
Most of the customs observed with regard to the events leading up to marriage in Thailand have evolved over the years to protect the prospective bride. They are intended to prevent the young woman being taken advantage of, and protect her valuable virginity until she is married. The sin sot is intended to offer the groom some form of security that this is the case, and is returned to him if not.