The primary differences between SSDI and SSI relate to eligibility. SSDI is available to those who have paid into Social Security through taxable income. SSI, on the other hand, functions as a safety net for people who do not qualify for SSDI and have limited financial means. SSDI is intended for people who used to work but can no longer do so due to an impairment, while SSI assists those with low income or who have not worked enough to earn the credits needed to quality for SSDI.