What is the use of realationship?
I am guessing you mean as in a "love" relationship. if so, it is to show your love and care for someone special.
For some, it's use is for attention. For others, it is important between them and their loved one.
You would be a lonely cat lady ( or man ) if you did not have love.. Lol
People fall in love?
I have yet to find out. I don't think I will know the answer until someone comes along to show me.