Logged QSO are stored not only in the main log, the data about the callsigns are also stored in the internal callbook. To use this information for quick tips about the name and QTH of the contact when entering a new QSO, you can mark 'Enable QTH and Name autocomlete in "New QSO" window' in the parameter settings window of the program.
You can register the information into the internal callbook without saving the QSO in the log. For this purpose there is  a special command Save to internal callbook in the context menu of the "New QSO" window.