I remember a time when you could shake hands over a deal and your word was considered good enough collateral.
Now we need lawyers and "terms and conditions" in fine print to cover your ass.
There was a time when vows meant you could trust the other person. Now we need to decide on prenuptial agreements and consider separate bank accounts before we agree to love and cherish "forever".
So what happened to honesty? Why is it so difficult not only to trust people but also to uphold the trust that people have in you?
Mortgages (the ultimate form of posting collateral), "indentures" describing business arrangements and even pre-nuptial agreements are centuries old (pre-nups are actually millennia old).
What you buy when you get a lawyer involved is three things, assuming the lawyer is competent:
(i) The lawyer's role is to think of unlikely scenarios arising under the agreement that would be very bad for you, but that you may be unaware of of overlook. Often as not these are low probability events, but the wealthier society gets the more money we can spend on lawyers who save us from these low probability, but high impact events, like a business partner who goes bankrupt or the question of who bears the loss between a buyer and seller if the third-party shipping company fails to deliver the good purchased.
(ii) The legal way of writing (though not necessarily legalisms in and of themselves) tends to be more precise than everyday speech. When there are problems and the other contracting party is taking a herd line position, looking to a detailed and explicit written agreement is better than trying to read between the lines and think back to your original expectation regarding the deal. Even if the other side's hard line is right, at least you "know", so you can be sure he is not simply cheating you.
(iii) Lawyers play good cop, bad cop well and generally advocate well on your behalf, even in contractual settings. They can be either cop. One of you takes a hard line and the other adopts an easy going "we're all here to work together" demeanor. Now, if the other side also has a lawyer, the advantage is offset in accordance with their relative skill, but that is to say, iif the other side has a lawyer, then you'd have a strong incentive to not be without one yourself.
Legal agreements and the legal profession in private business is a luxury that we pay extra for to get those benefits. The reason people in the past used them less was "cost" in the broad economic sense. It wasn't worth it to them--not because they were more honest than the wealthy (who have been using lawyers for centuries...first year Contract and Property law classes in the U.S. often start out which cases that are centuries old and English)--but because those benefits weren't worth the a large proportion of their meager resources that it would have taken to obtain them.