Learn About SS-5
The Social Security application form is formally known as Form SS-5. Though it has changed over time, it is generally chock full of useful information for family history research.
This is especially true in the early days of the Social Security program, where Form SS-5 was mostly filled out for adults (rather than for children, as is often the case today).
The Social Security application often includes:
Applicant's full name, and maiden name, if appropriate.
Complete address information
Date and place of birth
Full names of both parents, including mother's maiden name
Date of Employment
As you can see, there's a lot of wonderful information there that can be very useful in compiling a family history