If you haven't been together long and don't own much, it's really not necessary to make a written agreement. But the longer you live together, the more important it is to prepare a written contract making it clear who owns what -- especially if you begin to accumulate a lot of property. And, if you buy a house together, it's a good idea to create a property agreement.
If you don't create a property agreement, you might face a serious (and potentially expensive) battle if you split up and can't agree on how to divide what you've acquired. And, when things are good, taking the time to draft a well-thought-out contract helps you clarify your intentions.