A business contract typically tells a story about what two or more people hope to achieve by a mutual exchange of items of value in their deal. For most such contracts, a "meeting of the minds" lies at the heart of what the contract means and how it will be interpreted. If you don't have a formally executed document that itemizes the deal in total, it usually helps in documenting your intent if you have contemporaneous emails that set forth that intent.
Note a couple of points:
1. It is important that they be "contemporaneous" - after-the-fact emails that try to paint a self-serving picture do not help and can sometimes backfire.
2. It is important that they be clear. In emails, we sometimes ramble or make ill-conceived statements or simply incomplete ones, and all this can be twisted against you in any later dispute. Best advice here: be yourself and set forth the terms as you see them honestly, clearly, and in keeping with the nature of the relationship (no CYA stuff), preferably without legalese; don't document pointlessly but only as needed to add clarity to what you are hoping to achieve.
3. Watch out for "merger" clauses in any final documentation if your expressions of intent as set forth in emails are contradicted by what the final contract says. A merger clause says that the written contract sets forth the entire contract between the parties and that nothing that has previously been said or even tentatively agreed in prior exchanges between the parties is valid if it contradicts what is set forth in the final written contract. When you sign a contract that has such a clause, you will be hard put to push any contradictory interpretation that you might have previously documented in emails. The emails still matter if all they do is help interpret otherwise ambiguous expressions in the final contract but not if they contradict clear provisions of that contract. The lesson here: don't assume that, simply because you have documented something in an email, it will necessarily help you in the end.
4. Get good legal advice before signing any important contract and normally document it in a formal contract if it is at all important. If it is a smallish, then the ebb and flow and the informality of email documentation can work just fine but do be aware of the limitations of any such approach. A good lawyer will also help think through the issues and help eliminate problem areas concerning whether the contract expresses your true intent. Do not use a lawyer slavishly but use one when it matters. This is really the theme of my GP post where I essentially say be careful not to be penny-wise and pound-foolish when it comes to using legal help on things that matter.