There are several reasons one may wish to have a revocable trust in addition to a Will. The primary reason is to avoid probate. Probate is the legal proceeding in which the local court supervises the administration of the estate, the claims of creditors, and disposition of assets in accordance with the Will. Depending on where you live, it can be unnecessarily time consuming and expensive. Assets in a revocable trust are generally not subject to probate.
A second reason to have a revocable trust is to avoid probate in multiple jurisdictions if you own real estate in more than one state.
A third reason to have a revocable trust is to maintain privacy. Upon your death, your Will gets filed with the local court and become a public record. A revocable trust generally does not. If you would rather not have your wishes made public, a trust may be for you.
A fourth reason to have a revocable trust is if you are at a point in your life where you would like to have another person (your co-trustee) available to manage your assets for you.