Can not help with arousal in a couple and improve sexual relationships?
How does it affect relationships in general?

Definitely can, for me personally, that's what it's for, I smoke with my partner sometimes and I can say that it has really brought us very close together.
It helps you understand your partner better.
I can also advise you to read this article https://www.smokeandpoke.com/blog/can-weed-make-you-horny/, it says a lot about it.
I can add myself that under weed you can work through various psychological traumas with each other.
Weed is a great way to get to know each other, it will make it much easier for you to share very personal things, thereby getting to know each other better and better.

A lot of people neglect it and think it's not that important, and then they get trapped.