Some random questions on why a publisher may be needed. A publisher is not needed to ship games nowadays, but they can certainly help you get through a lot of unknown work, especially if you have not shipped games before.
Just random thoughts, a publisher can offer these services that you would not have to do yourself:
* Localisation services - to translate / record localised voiceovers in different languages.
* Certification and compliance - testing and guidance through cert processes on various platforms and regulatory bodies (GDPR in Europe, for example, if you are gathering telemetry data)
* Marketing, brand, social media management and engagement - anything from community management, to organising press and review events, to handling social media accounts, to doing local or online marketing, market research
* Sales related services - distribution contracts for physical copies, localised materials and so on
* General testing and QA services - for the game clients, for backends and whatever else your game may have
* Publishing services - handling all of the complicated publishing processes, especially on consoles
* UXR services - user studies, early playtests, feedback sessions
And many other things.