Thoughts on friends with ex

What are y’all thoughts on being friends with your ex?

Edit: a lot of my friends are friends with they ex’s and then just go on and off with them. Every person I know says they will got love for they ex and want to be friends. But for me after I stop talking to you I’m just like fuck your eat a dick then I move on. Idk how people be friends with them. I just feel like that’s where most people start toxic relationships. What’s y’all thoughts on what I think?