Mormon belief is that baptism is one of the few strict requirement for entry into Heaven, but that it can also be performed for the dead by volunteer proxies. Even the most obstinate sinners and nonbelievers will gain entry into the lowest-but-still-pleasant level of Heaven if they haven't committed the unpardonable sin[1]... so long as they've been baptized, in person or by proxy. Combine those premises and it becomes an obvious moral imperative to gather as much historical genological information as possible to perform proxy baptisms for as many deceased people as possible.
Of course, this presumption of religious axioms also has a history of upsetting people of other religions, and the extensive information-gathering involved often uses borderline unethical methods, such as the various geneology research service websites that are run in one way or another by the LDS Church and include terms of service that let them use the info you piece together however they like.