What is the role of Honeymoon vacation after marriage?

It's a nice way to start off married life, getting used to being a husband or wife, away from the usual everyday same old same o! And a way of relaxing after all the stress and trauma of the actual wedding. I think it's probably one of the most romantic times during the marriage as you both maybe pay more attention to each other having so much time together on your own. It's also a time when other holidaymakers, who hear you're on honeymoon, will say "awwww" and leave you alone as a couple rather than perhaps try to butt in and make friends with you! I only say that because on the first evening of our honeymoon, one couple were seated next to us at a dinner table and we got along so well that we continued to drink together for the next 2hrs just getting to know each other. When we eventually we'd only got married the day before, they felt really guilty and promptly made their excuses and left, despite us assuring them if we didn't enjoy their company we'd have excused ourselves earlier! They said they felt they had "gatecrashed" our special time! But I must admit, it did feel like a special time, but not one that I'd necessarily want to spend ONLY with my husband 100% of the time.

Nice question...maybe to relax after the organisation of the marriage, or to relax before starting a stressing life, because start the life together is not easy....

It traditionally was a way for couples to get to know each other intimately without the interruptions of everday life. It doesn't mean as much now since most couples sleep together before getting married.

it helps the couple to know each other better and spend quality time

A time to spend by yourselves, with nothing from your regular lives to bother you as you get to know each other better. Oh, and a rest after all that wedding planning!