Email Threads on a particular subject is mixed with threads from many other diverse subjects in the inbox, making it hheard to search and follow up. Topic threads are part of a topic, making them easy to search, follow up and respond.
Collaborating in email opens up the risk of spam and phishing. In addition email targeted for one partner ornazation risks ending up in the inbox of another partner, making it less secure for cross organizational collaboration.
Messages posted in a thread are real time meaning they can indicate status such as number of unreads, new messages and so on. Whereas email messages do not have real time communication features like those.