What's the purpose of false memory?

I'm dealing with this now and realizing my false memory is physically impossible for it to have happened logistically. Anyhow, trying to figure out what the purpose of false memory is.

I know for myself I was dealing with quite a bit of shame and guilt around sex and sexual desire and this false memory came up.

Also, when it comes up, false memories, do they just show up out of nowhere? Like, I know I was dealing with a stressful time and that's when it showed up.