The financial justification was probably as a way of buying users:

> Users will have an opportunity to migrate their bookmarks to a Pinboard account, which costs $11/year.

It's also possible maciej wants to ~~do Machine Learning on the data~~, as they have "over a billion bookmarks", but that seems unlikely to me.

I'd bet the "real reason" truly was sentimental:

> Delicious ... is a fascinating piece of web history. ... I bought it in part so it wouldn’t disappear from the web.