Compared with
A shared document, and this
Almost every arrangement between two people ends up in a shared doc or a message thread, because everybody already has one. It is a reasonable place to end up. It is a difficult place to work things out.
What a shared doc is good at
Being there. No account to make, no explaining to the other person why they should install something, and a link that works on every device either of you owns. For writing down what you have already agreed, it is hard to beat, and nothing here is trying to replace the moment where two people put the arrangement in writing.
Everything in it is visible the moment you type it
There is no draft state that the other person cannot see, so the first sentence is written for an audience. That is the whole problem: the thing you would have said if nobody were reading is usually the thing worth saying, and a shared cursor removes the possibility of writing it down at all.
Here, what you write is private until you decide otherwise, and Independent Perspectives makes that structural: both accounts are written before either is read, then opened together.
There is no structure when it heats up
A document does not notice that the last three edits were made in anger, or that you are having the same argument as last month in different words. A thread has even less: whoever writes last, wins, and whoever is angriest writes most.
The record is buried in the argument about the record
In a thread, what was agreed sits in the same place as the forty messages disagreeing about it, and six weeks later neither of you can find it. In a doc, the agreement is a paragraph somebody edited and nobody can now reconstruct.
Here the agreement is a separate thing from the discussion that produced it, it records who agreed and when, and it can be exported and taken away.
Nothing is deleted for you
A shared document keeps every version, including the sentence you wrote at midnight and removed at seven. Depending on the platform, that history may outlive the relationship, and it is generally visible to whoever holds the link.
When the document is the right tool
When you already agree and need to write it down. When the other person will not use anything else, which is a real constraint and not a failure. You can export from here and paste it into whatever they will open.
What this is not
Not a legal service, and an agreement recorded here is not a contract. OnBetterTerms is a private workspace for reaching an understanding and writing down what it was. Where something needs to be enforceable, that is a job for a solicitor.