To not understand why people get married?

Marriage mitigates the financial risks of pregnancy, childbirth, and child-rearing. Most of those are incurred by one person in the partnership, but not always. Even if the woman incurring the risk is the higher earner, statistics show that having a child will negatively impact her earning potential and she is taking a risk that she could lose her job entirely by becoming disabled via pregnancy.

I wouldn’t have a child with a man that wouldn’t marry me first. It would signal to me that he did not care about my financial stability and expected me to be the one to shoulder the risks of parenthood.

In other circumstances, like if a couple is not going to have children or if they are older and already have children, the rationality of marriage changes.

I wasn't protected by being married. I have my kids 100% of the time (the circumstances make this necessary), my career is in ruins as I can no longer put in the hours and I had to pay my ex a settlement which involved starting from scratch with the mortgage. I will likely always be "poor" now as I don't have time to catch up later due to my age. If (and it's a very big if) I ever meet anyone else I won't be getting married again.

No tax is due on transfers between spouses.

Hint inheritance tax

The lower earner wants to have access to the higher earner's income and assets on break up.
Women who want to have children and stay at home with them and want the same as above as they'll reduce their earnings by doing that.
Seen as showing commitment.
Tax when older.
Etc etc

Waste of money and merely a show to the world

Waste of money is purely subjective - if someone spends it and doesn't regret it, is it a waste? But even so, you don't have to spend a lot, I got married for a few hundred pounds at a registry office.

If you think it's "merely" for show, you don't understand the legal consequences. If you think the legal consequences are not what you want, then that's fine. But is it very clearly not just a show, it has real ramifications.

OP, in a society where marriage is no longer required to be included in most things, it is harder to justify. A single person today can have:

a career
a home of their own
manage their own finances and qualify for all govt benefits
have children
can cohabit with whomever they choose

So it comes down to the concept of making a lifelong public legal commitment, although it isn't life long, and a spouse can wander off whenever they like.

Inheritance tax provision is better for marrieds
Management of wills, shared ownership of assets, having a say in the care and funeral of a spouse.

It also has the downside that it allows another person to sink you deep into debt without your knowledge.

Tax, money, next of kin, parental responsibility, love, security.

As usual the first response nails it. Legal protection is the key. Everything else is just dressing.

You can get married for a couple of hundred pounds at the registry office. We’re married because I would never consider having kids without being married. Those costly divorces give protection to the partner who may work / earn less in order to do more childcare.

You don't have to spend that much money or make a show to the world but people like to make a legal and public commitment to each other.

More practically people get married to protect the parent who reduces earning capacity to care for any children, gain next of kin rights and to avoid inheritance tax.