Why does my ex want to be friends with me

My girlfriend dumped me in April after we had been in a relationship for 3 years. We have known each for 7 years and we were best friends before we got together. Even though I have discovered she now has a new boyfriend she says that I am her best friend and she wants me in her life. Our relationship was a really good one. The reason she broke it off was because she had moved away and we could only see eahc other at weekends and also she was struggling with the change in me ( I was struggling with anxiety/depression but had not been diagnoised). She didn't talk to me about issue and instead called it off. Since April we have remained in contact and I am now living in the same palce as her again. However she is now seeing someone and I am confussed as to why she wants to remain in my life. I have asked her and she said it was beacsue we were best friends before we went out together and she loves my company and thinks I am a wonderful person. We have only had one night out since I moved and that resulted in her telling me that she has moved on and closed the door on us, but we ended up together that night. Any advise on why if someone has fallen out of love with you wants to still be your best friend and see you weekly??? It was a same sex relationship and she is now with a boy