Actually, you don't. However, since it costs you nothing, unless you receive benefits (i.e. you win,) why would you risk losing? Our experience and knowledge of the issues and Judges, puts you in much better chance to win.
In order to even qualify for disability benefits, you must be unable to be substantially gainfully employed. You must have been employed for must have accumulated at least 40 quarters of work in "covered employment". A "quarter of coverage" generally means the three-month calendar quarter. In addition, you must earn at least $1,320 in a quarter (in 2019) for it to count.