A will instructs your personal representative how to distribute your property after you pass away. Your will nominates who you want to represent your interests after you die. This person, your Personal Representative, collects assets, pays your bills, and distributes the remaining property to the persons you specify in your will.
It's important to have a will, even if you also have a trust, to ensure that any assets that do not get assigned to the trust can be probated and placed into the trust for distribution. Of course, better is to ensure that, through proper trust funding, there are no assets to probate.