Sometimes claimants have not filed both SSI and SSDI claims. This can be significant if the date last insured is before the date of disability. For example, let's say a SSDI claim is pending, but the date last insured is December 31, 2007. However, claimant doesn't have evidence proving disability before 2015, and there's no SSI claim pending. This means claimant would be out of luck if no SSI claim were pending. BUT, if claimant files a SSI claim before the hearing--even if it is on the eve of hearing--the SSI claim will be be "pending," and it can be considered at that point. This can make all the difference in the world between winning and losing certainly in this limited hypothetical and in other situations.
For more information on Social Security Disability, visit the Social Security Disability claims website operated by the Social Security Administration.