Clean out the lists, remove spam and duplicates, set subscriptions, set custom fields. Like, customers who bought certain products could be put onto lists for those products. Completing information that may have been missing on subscription. I like to work in spreadsheets for this. Data can easily be filtered, sorted, mass edited, combined with other sheets etc.
In the past, I have sometimes edited data directly in the DB, but that's always risky, and extra effort putting the queries together. (Have not done that in this particular DB!)