Letters are exchanged between the scammer and victim until the scammer feels they have groomed the victim enough to ask for money.
This might be for requests for gas money or bus and airplane tickets to travel to visit the victim, medical expenses, education expenses etc.
Upon finding victims, scammers lure them to more private means of communication, (such as providing an e-mail address) to allow for fraud to occur.
The fraud typically involves the scammer acting as if they've quickly fallen for the victim so that when they have the opportunity to ask for money, the victim at that time has become too emotionally involved, and will have deep feelings of guilt if they decline the request for money from the scammer.
Also because military public relations often posts information on soldiers without mentioning their families or personal lives.