Your access to a popular and commercial fork might be non-existent. You would maybe not even mind that they’re selling your code - possibly just wanting to tune it a bit, since you know how it used to work. But the fork can decide they’re proprietary and they won’t give you any source code.

Or from the other side, GPL would guarantee that you yourself (and your project’s contributors) would still have access even if a good fork emerges.

Things happen over years or decades.