"I love you but I'm not in love with you", what exactly does this mean?

So, you love me like a friend, a sister, a mother or what exactly? And if so,then why would you start a relationship with someone that you love in that way? Or could this just be an old excuse that people use when they’ve fallen out of love with someone and want to put an end to it?

Just curious what people really mean when they use this phrase to break up with someone.