Y-DNA (Y-STR) testing is essential for genealogical research. However, results are useful only when compared to others, which is why a person needs to join or start a surname project.
The most important use is assessing whether lineages in a tree are plausible. If someone does not closely match the others in a tree, either the paper trail is wrong or there has been a non-paternal event (NPE). We found two such cases.
It also focuses the research on orphan lines by paring down the number of families that might connect. This aspect was where the biggest successes in our project occurred. Researchers worked for decades to find connections of two families to one of the documented colonial families to no avail. DNA quickly gave us the answer: They were not related.
You may also uncover a very useful branch-defining mutation. This is one that occurs early on a very slowly mutating marker. The possibility of finding the same mutation in a different line (parallel mutation) means that faster markers need to be used carefully. We were lucky to uncover a mutation on DYS438, a very slow marker, that a family progenitor had, so we can readily identify members of this family by testing.
Another use is grouping people within a surname who match closely. This answers the question of whether all people with a low-frequency surname are related and has some networking value. Once in a while people will connect with someone who has an older tree. However, the DNA itself will not help extend the lineage. In projects that do not work with trees, the results can be spotty. Many people have joined them and been very disappointed with how little they got out of testing.