As @Delapouite said on Github:
The added value over IRC is that you can post long messages, with code highlighting, links, tags…
Mailing lists are always a mess in comparison.
I will add other benefits include images, formatting, mobile interface, web interface, mailing list mode, rss feeds, time-shifted (not real time like IRC), embedding and history.
Another set of nice features is being able to reorganize posts (I recently reorganized @eraserhd’s awesome golf thread into two threads then deleted the original), and polls as seen on the Future Assistants post.