When you fail to reduce your agreement to writing, there is the very real risk that you may be out of luck if the other party to the agreement breaches their end of the deal. And the reason has a name:

The Statute of Frauds.

This doctrine requires that certain categories of agreements must, as a matter of law, be reduced to writing. And if they aren't, a subsequent claim that the agreement was breached will likely be dismissed as a matter of law.

As with most rules, there are some ways to try to circumvent the Statute of Frauds - even if you failed to reduce your agreement to any form of writing. Perhaps the most practical way of doing so, however, is demonstrating that despite the lack of a writing, a significant part of the agreement had been performed, whose only real explanation would be that these actions were taken because a clear agreement had been reached. 

In other words, if the actions that were performed were "merely steps taken in contemplation of a future agreement," that will not suffice to take a case outside the ambit of the Statute of Frauds. See, e.g., General Obligations Law 5-703; Messner Vetere Berger McNamee Schmetterer Euro RSCG v. Aegis Group, 93 N.Y.2d 229, 235, 237 (1999).