Money. Arrowhead has done a few other (very cool) games before Helldivers 2 (HD1, Magicka) but AFAIK nothing that was popular enough to allow them to finance an expensive product like HD2 on their own. And the success proves them right: ignoring the PSN shitstorm the games is highly liked and got a lot of players. So they where able to use that money to actually build a successful product. That’s what publishers do (or one of the things): financing of projects the developer itself cannot pay for alone.

Quite a bit, believe it or not. It's often an economies of scale problem, as well as a subject matter expertise problem.

On the front, working with a publisher can help with funding. A publisher will often times front a fair amount of development costs, which depending on funding sources will often time make up a significant portion of the revenue needed for development. When people see exclusivity deals, that happens because the developer receives advanced funding for development costs, usually a significantly larger amount than one would receive from a publisher that has no platform associated with it.

For smaller studios, these teams may not have any marketing expertise. While an independent marketing team may be able to help out with this, the publisher managing this relationship, especially with teams either internal or external that they have experience with, allows the studio to focus on their development efforts.

That's not to say this is always sunshine and rainbows. The stories of publisher imposed deadlines, restrictions, etc. are true, usually without a lot of the inside baseball color of the why of these deadlines are required. That said, when Other People's Money comes into play, things always change. The cost of delaying a release is not small, even more so when there are physical products being made and fulfillment to be managed. That's not me being an apologist, just trying to add a bit more color to the social media talking points.

To the point I think you're trying to make though, with the amount of self publishing tools available, it's definitely not the necessity that it used to be to be. However, the dynamics of whether one can do something, vs. whether they're the right ones to do so something are always nuanced, especially when it comes down to the calculus of whether the benefits and the trade offs line up.

Publishers can be a massive boon for developers.

They provide a huge advertising presence for one - it's hard not to stand up and take notice when a big company like Sony plasters their front pages with ads about a game.

The other benefit is money. Smaller studios often don't have the up-front capital to outright create a game on their own - a publisher can be that stepping stone.

Well, in this case Sony funded development on HD2 for about 8 years. That's usually the value they bring.

Because publishing requires contacts and resources. Small developers don't have either.