I formed a PBC and worked at a well-known PBC. Personally, I opted for a PBC because I liked that I could balance a specific cause with shareholder benefit.

In most cases, it doesn't really matter. The board + management is still in charge, and they have significant legal leeway regardless of the structure. But there's little additional cost to opt for a PBC, and it does give you more legal defensibility to be truly mission driven. Standard C Corps weren't really intended for mission driven companies (see the shareholder primacy norm).

I think it's popular for AI startups, because many great researchers understand the risks involved, and they don't want what they build to be controlled solely for shareholder benefit.

While non-profits are also an option for a mission driven org, it's harder to raise the large amounts of cash that some AI startups need, and laws around deferred compensation and private inurement (e.g. options-like structures) make employee compensation harder.

The important part is PBCs protect you from a shareholder primacy directive. Eric Ries describes it in his new book, but the example he gives is if the most evil company you know tried to buy out your company you have to do it in a normal "best practices" C corp because it is your fiduciary duty. PBC helps prevent that based on your declared mission statement. If the sale doesn't facilitate your mission then you aren't obligated to sell.