Congratulations on your 21st wedding anniversary @bunnypuncher.
Answers to the question in the order in which it was asked:
Marriage is the oldest institution known to man- I stand to be corrected though, families constitute the society and without marriage the family will be nonexistent. The marriage institution is the thread that stitches the fabric of the society together. Within this institution, children are raised to understand how society works; respect authorities-this is achieved by imbibing in them respect for parents, obey laws and regulations - this is achieved by making them respect their parents, etc. These, among a host of other things shapes and prepares children to be law abiding citizens of the society. Most, if not all societal miscreants are either products of broken marriages or born outside wedlock.
Marriage bonds two people together, they're committed to each other and together they make their lives better. Like my favorite book says, "one shall chase a thousand and two shall chase ten thousand". The institute of marriage is still relevant in our society today like it was thousands of years ago.

Absolutely, people in a relationship should get married. No matter how long a couple dates or live together, regardless of whether they're sexually intimate or not, without marriage they're just two individuals relating on an emotional level, without any real commitment to each other. Marriage validates whatever commitment they have. Without that validation, either of them can just opt and leave with no bond to stop or prevent it. Marriage is the key to starting a family, otherwise it will just be two people with child(ren).

The institution of marriage is being flooded with a river of abuse. People get married for so many terrible reasons like the aforementioned, which are a far cry from what marriage was originally intended to be; companionship and family. No matter the reason for wanting to marry someone, if it doesn't include the aforementioned, then the reason or motive is wrong.

That piece of paper is a legal proof of commitment as couples. Like I said earlier, marriage is an institution and it is recognized by the state or the society. It doesn't matter if your wedding was a carnival and the whole nation was in attendance, if that signed piece of paper isn't involved, the marriage is null and void. That piece of paper protects either party in a case of death, it is a evidence that everything owned by the deceased belongs to the living spouse.