I think knowing you will marry someone is different to knowing that this person could potentially be the one. After all we don't always get to marry the one.
For me, I met my husband in 2013 and we were very on and off for a couple of years. He never thought for one day (for a long time) that I would be his wife, as we were so different and argued about everything. I knew deep down that he was the one for me but I also never saw a happy ending for a very long time as there was so much hurt and pain between us both.
We broke up in 2015, I dumped him actually and 10 months later we somehow got back in touch because of some personal circumstances that brought me back into his life. It was like the time apart helped us both understand who we were as individuals and what we did wrong before in the relationship. More importantly that time apart helped my now husband understand what he did wrong and what he needed to change. We were back together a year and a half and he proposed. Then just a few months ago we were married.
I realise now that if we had got married during those firs 2 years of constant problems, love and fights, we would never have made it. You really have to know someone to take that step and realise that marriage is about more than love. Its about commitment, trust, staying when the going gets tough.
Today, me and my husband could no be more happier. We are trying for our first child and we never fight. Even we do have problems, he makes me laugh after 2 minutes. We evolved from being two totally different people to being one soul joined by to bodies.