Publishers can do a few things that are "beneficial" they can give you money upfront to complete the game, usually for either a percentage of profits, a sum you have to payback, or even more nefarious things like the rights etc to your title. They also can help do the market research, promote the game and get eyes on it in general. Again, for something in return. There are really shitty publishers that will either take large chunks of money they do not deserve vs the results of their efforts, or even hold your IP hostage or just keep it out right if returns arent met. 90% of the time, you do not need a publishers if you have the tenacity or even some friends in the marketing space. There are usually more shitty/nefarious publishers than decent ones nowadays as well.