To me, if I'm going to marry someone, I should have all of the information beforehand. I want to live with them before even getting engaged. I want to know if I can live with this person for 60 years, can I deal with 60 years of awful snoring? Or 60 years of someone who has a different expectancy of house cleanliness than I do? Or 60 years of vastly different political views? Could I deal with someone who only wanted to have sex once a week while I wanted it 5 times? What if my prospective husband was a Trump supporter? That would tell me that we have hugely different values. What is he thought that getting drunk every night was normal? Or had a gambling issue? Or wasn't nice to animals?
Sure, I could deal with one big issue, but not 3 or 4. I could deal with doing all of the cleaning if he was taking care of all of the accounting, etc. I could deal with snoring if ....etc. But I really think you have to have all your information before you get married so a few years down the line you aren't glaring at your snoring husband, googling divorce lawyers.
And sex is an important part of a relationship for me. It is a big part of affection and intimacy in a relationship. There are subreddits dedicated to people who have mismatched sex drives and needs, and are in unhappy relationships because of this. I know what my needs are, I know the frequency I need to feel loved and fulfilled, and if I can't get that from a partner I'm going to be unhappy. I'll feel unwanted and unloved and rejected, and that's the opposite of what marriage should be, isn't it?
The point of living with someone before engagement, having sex before engagement, going through a stressful time beforehand, talking about what's important to us and seeing what we can compromise on, is to see if I could happily live for 60 years with this person and go through all the bad times but still come out the other end in love. Love isn't magic, it won't make two people who are vastly incompatible work for the long term. So I was taught to "test drive" relationships before getting married so the marriage would be a forever thing.