I guess it's depend on your goals and what kind of game you are buidling:
* Funding. Making games is hard and while some of them can be finished using limited resources and free time. Having even small salary is better than living off kebabs and ramen (depend on location). Also artist dont usually work for free.
* Expertise in limiting your creativity. If you actually want to finish and release something then having deadlines and feature cuts is actually a good thing. Yeah most of these 1000 cards on "Ideas" list wont be implemented, but you'll get something done.
* Marketing. Making a good game is not enough nowadays to get any return on time and money invested. Bare minimum to launch on Steam is to collect 10,000 real wishlists before release since otherwise your game wont be features. Wishlists alone is a difficult task that require deep know how in traffic arbitrage since otherwise you'll spend 5x more money. Also making good marketing
* PR. Even in a team of 10 project management and coordination take a lot of effort. Dealing with press, youtubers, streamers and possible future community is a hard work that might require more capacity than you have.
Yes you can be self-funded and everything like marketing and PR can be done in-house, but it's cheaper for publisher since they usually have dedicated people working on each area full-time. It's doable to make it all yourself, but every unique role will distract you from actually making fun game.
Also publisher that invested money into your project will also be motivated to at least get that money back.
