I can't speak for all writers, but here are a few reasons I have seen some authors going with a trad publisher:
* Reputation. It can just feel cool to say "Oh yeah I have a book published by Tor" (or whatever). This one is pretty weak for me. Trad publishers don't hold that much special prestige anymore.
* Translations. There are some great untapped translation markets out there (like Germany). Some authors self-publish the English version of their books and sell translation rights to a publisher. The publisher then does the work of translating and republishing in the target countries, taking that effort off the author. The royalties are lower, but funding high-quality translations can cost a fortune and for many authors offloading that cost and effort can be worth it.
* Audiobooks. Similar to translations. Author may publish the ebook themselves and sell audio rights. Good narrators can cost a fortune, and many authors can't justify that outlay themselves. A trad audiobook publisher can get access to the best narrators and fund the entire production if the author doesn't have the means or desire to do it themselves.