There are two situations:

If the game is almost developed or already released, the developers can get support from a publisher to aid in marketing and distribution. It's uncommon for a development team to have any marketing talent so it's best left for the experts at the publisher. Distribution also means help in porting the game to different platforms like mobile and console with a separate development team. An example of this is Stardew Valley that had great success on PC but needed publisher help for porting to other platforms.

The second situation is where the publisher helps fund the game development. You can take a bank loan to fund your own game but you will be on the hook to pay off that loan. If the game doesn't do well, you might end up homeless and poor. Publishers take on that risk when funding developers. If the game does poorly, at least the developers were paid for making the game. In return, the publishers have some control over the development process for good or for worse. The developer also has access to the publisher resources like in-house game engines, knowledge/talent, and QA teams during development.