The primary purpose of a Special Needs Trust is to use your trust assets to pay for items that are not provided by Supplemental Security Income (SSI), Medicaid, or other similar government-sponsored programs.
If you only had a will and left your special needs beneficiary an inheritance above the limit required to maintain government benefits, that beneficiary would be disqualified from receiving those benefits. A Special Needs Trust allows your beneficiary to continue receiving government benefits while supplementing those benefits with the assets you leave for them.