What is the scientific purpose behind "romantic" attraction?

I think it's for a man and a woman to stick together for the sake of raising children, to ensure their survival into adulthood. This is just my theory, but it is possible that in caveman times, once they became of age, every young man and woman would have sex with lots of other women/men from their tribe, before settling down with one of them (the one that they're romantically attracted to). Soon afterwards, the woman in the relationship would begin to show that she's pregnant, but whether she's pregnant with her partner's child, or with the child of one of the many men she's had sex with before her partner, would be unknown. Regardless, neither of them would really care. Although the man in the relationship would be well aware that it may not be his biological child, he would still treat it as his own child, since it's the child of his life partner. He wouldn't feel duped into taking care of someone else's biological child, since someone else may very well be taking care of his biological child anyway, so that would balance it out. As long as it was from his own tribe, it was ok. It's only when civilisation began, and therefore concepts like royalty and bloodlines began, that people really started to care about whether their children were their biological children.

Well there is both sexual and sensual attraction. A person who doesnt desire sex may still want companionship and non-sexual intimacy. What exactly is the meaning of 'purpose' here? Biologically? Who knows, I guess it exists because humans are by default social creatures and feeling romantic love creates the urge to share a life with another person and looking after each other.

Alongside normally having a sexual component it serves the same purpose as platonic love, it promotes looking after each-other, co-operating and being altruistic. We are descended from social apes and are supposed to support each-other.

It is sexual stop asking the same question