For an extensive explanation of the mailing feature in SoSci Survey see Send Mailings.
The short version: You import email addresses (anonymous or not), send one or more maillings to the adressees (containing a personalizes questionnaire URL), and then you have a person ID (SERIAL) in each data set. This is particularly useful when conducting multi-wave surveys. And even more so, if you want the data to remain anonymous.
Privacy note: If another project imports the same email adress (or if you import it twice), a different person ID will be assigned. When the address entry is deleted, the association between person ID and name/email adresse is deleted as well.