What are some benefits of posting on the Lua mailing list instead of
places like stackoverflow?
This response is not specific to Lua.
If you have a specific problem for which you need a simple direct answer without much discussion, StackOverflow works well. Most likely your problem has been posted before and can be found with a quick Google search, and hopefully has a useful answer. If not, post a question and you'll probably get an answer. But you might miss some of the nuances by relying on Stack Overflow.
Mailing lists replace direct Q&A and simplicity with discussion and nuances. Post your problem to a mailing list and it may take longer to get a response and you may have to wade through a dozen replies, but you get more discussion about the theory behind the solutions and the nuances of why one approach might be better than the others. Mailing lists are also better structured for back-and-forth discussions and debates on the merits of this or that, which are discouraged on StackOverflow but lead to understanding the topic more thoroughly.
In the end, you get quick and simple answers from StackOverflow, but you get better answers and greater understanding from an appropriate mailing list.