We're still learning the details of it, but it seems that the mass registrations are a sort of "setup" step. They're the fuel that can drive the fraud in various ways.

Firstly, if you're just registering voters that's fine and dandy and they do target liberal areas. This serves as the natural, perfectly legal cover for the operation.

Secondly, as you collect voter registrations, you can record those names. Now you have a database of newly registered voters. What makes that data valuable?

Well, they also have access to the ERIC system (which is used to clean voter rolls). Why do liberals fight against having the voter rolls cleaned?

Because the fraud is driven by cross referencing newly registered voters (who likely moved), with ERIC rolls, to identify plausible sources for votes. They can then vote the person's old address without being noticed.

TLDR: They use data from voter registration to identify areas where they can covertly cast ballots. The more churn and invalidity there is in voter registration, the more fuel they have for fraud.

Basically an illegal registration is even better than a legal, because no legitimate voter is ever going to conflict with the illegal poll book entry.