What is the purpose of snowball sampling?
Snowball sampling can be used in a number of circumstances.
Sometimes the researcher may be able to identify only a small
number of individuals in the target group but these people are
likely to known others in the target group, and people that they
know will also know more such people-and so on. By including those
few initial people in her sample, and asking them to recruit
others, the researcher may be able to achieve a useable sample
size.
Then, sometimes, a researcher is actually more interested in the
connections between people, or in other words, social networks. In
this case, snowball sampling can be used to identify those
connections.
Please see the link.