Here is a typical scenario, especially in the Bay Area start-up world: a young start-up engages in hiring discussions with an engineer or a marketing person. The employer is in a hurry to hire the right person to get their business up and running as soon as possible, and the candidate is also anxious to get hired. They begin discussing the details of that employee's equity rights. They go back and forth by email a few times, running numbers and percentages by each other, trying to reach an agreement. Then, as that employee comes on board and becomes focused on and consumed with his work, that conversation dies down for some time. It's not clear what they agreed upon if anything. Earlier, the start-up CEO might have promised to give that employee a formal equity plan as soon as possible but never followed through, getting caught up with dealing with a hundred other things that need to be addressed at a young company.
Then, one day that employee leaves the company for one reason or another. What equity is he entitled to, if the parties never signed any stock agreement? An employer cannot say that he doesn't owe that employee any equity. If there is some written communication regarding equity, then at the very least the employer usually owes that employee the lowest amount of equity they offered during those early negotiations. But how and when should that equity vest, and how and when can it be exercised? What about all the other relevant terms of equity grant? Absence of written agreement creates many such questions that can and often do lead to litigation, that could have been easily avoided if the parties had a clear, written agreement in place regarding equity rights and obligations.
Therefore, whether you are the employee who expects equity to be part of his compensation, or whether you are an employer who plans to compensate his employees in part with equity in your company - it's in your best interest to have a written and signed equity grant agreement in place as soon as possible.