There's a number of reasons, but our groups implementation is the most compelling one for me:

A threadable list can be divided easily into subgroups with their own subset of the list membership. Any member can create a group, and you can move threads between groups, even after they start.

In practice, this means you don't have to anticipate which groups to create in advance. You can just let discussion happen, and when critical mass around some topic is reached, move it off into a group without the overhead or interruption that comes from making a whole new mailing list.

This leads to more organic discussion, and also makes it possible for group members to help one another determine relevance. It's sorta like crowdsourcing your mail filters from the rest of your team. We've noticed that people are less likely to filter a Threadable list into another folder, so they participate more.

It's also more friendly to newbies, since there's less worry that a thread will end up in the wrong place. If it does, you can just move it.

As someone trying to work to get my lab to migrate our lists from Google Groups to Threadable (I've used both and really want to see my group switch) it is mostly that Google Group's UX is infuriating and using it often makes me want to throw something out the window. Also I miss not having threadable's groups features on my google groups lists, where we have to create a bunch of different separate google groups lists for various things in our lab. With threadable we we could just have one list and a bunch of different subgroups.