There are good sibling replies answering your question, but to give a specific example: if your application writes something but then doesn't need it anymore. Since we're talking about go, perhaps a data structure you need also holds a reference to data you don't need.

When the kernel needs memory, it goes hunting for a page it can discard. But since that's transparent, the kernel can only discard a page if it knows it can get it back (after all, it's still got valid data on it, and maybe you'll try access it again later).

If there's swap, a page full of stale/unneeded data can be written out to swap. But if there's no swap, your page of "dangling data that you'll never use, but is still valid & referenced" can't be discarded; the kernel doesn't know you won't want it later, and it can't recreate the page if it throws it away.

So like sibling said, at that point it has to find other pages it can evict from memory, ones that _do_ have somewhere persistent they can be written out to. Pages loaded from binaries on disk satisfy that, so those will get dropped instead.