It’s a big question these days about whether you should get one or not, but they traditionally do a lot of things that game developers may not be great at:

• marketing (this is a huge upside as marketing is time consuming and expensive)
• storefront management and relations (many publishers have connections with people who control digital storefronts so they get special treatment)
• resources for developers (some publishers serve as resources for less experienced developers if they need additional help)
• community management (this kind of falls under marketing, but some publishers have staff that manage outreach and community platforms)

I’m not an expert on this so anyone who knows more please expand on this or correct me if I’m wrong. This is what I know as someone who has looked into publishers for my own games but ultimately didn’t use one.