A lot of companies use Tableau to visualize their data. In the future if they want to use Tableau they (speculation) might have to host their data in SalesForce and/or be directed to use a number of other SalesForce features if they want the latest coolest dashboards. To me this acquisition seems be a play toward growing customer usage of SalesForce licensed functions and data migration to their cloud, rather than based on the current revenue stream. The lock-in is strong.
SalesForce has been pushing Einstein Analytics recently. I haven't used it, but I do see that moving an organization from Tableau to Einstein has a lot of costs involved so this would be a hard sell in many places. Having them both under one roof means they're able to bring a bunch of people across to their cloud and now that license revenue year over year is theirs with the additional data lock-in.
As someone that really dislikes vendor-forced lock-in and generally dislikes the way SalesForce controls your data, rate limits, maxes, licensing, etc, this move is about even more control up your stack that will seem like a "no brainer" to decisionmakers, which is dangerous. That said, I'm sure it will work well for some organizations.
EDIT: I would also love to see them spit out D3 or other open visual but then they'd be losing control of the secret sauce and the requirement for a license. Not sure there is an incentive to go that route.