Well, they do answer this question on their home page:
How is Capitalize free?
If you choose to open an IRA with one of the preferred partners on our platform then we may be compensated. This helps us keep our rollover service free for almost all of our users and allows us to provide enhanced levels of rollover service for transfers into those partners. None of those providers are ever allowed to impact the content on our site. If you’ve ever used a site like Nerdwallet, Credit Karma, or Lending Tree then you’ll be familiar with this model.
We also offer Capitalize Plus, a premium, paid service for users who want even more service & benefits, or for users who want to roll over into an IRA provider that isn’t currently a Capitalize preferred partner.
I could see where this could be useful if the 401k's are forgotten, or have been through several acquisitions. If you know the current whereabouts / account numbers of the old accounts, I don't know that this would be much easier than just initiating a rollover with Vanguard directly. Does Vanguard itself say anything about using them?