ANSWER: The "Supervisors Statement" is a required form which is nearly identical for both FERS and CSRS employees. It requires the supervisor to provide specific information about your performance, attendance, and behavior on the job. It is assumed that these are reliable indicators of whether you are unable to render "useful and efficient service" and thus whether you are considered disabled from your job.
The importance of the "Supervisor's Statement" varies with the nature of the case and with the individuals at OPM who review it. In short, its impact can't be measured. Since it could be important, you want it to be as much in your favor as possible. To that end, you should encourage your supervisor to provide as helpful a response as is possible. However, even if your supervisor did everything possible to be unhelpful, that would not necessarily prevent you from gaining disability retirement.
If you have previously received satisfactory or excellent evaluations in spite of your declining performance, your supervisor may worry about suddenly having to downplay your performance for the purpose of the disability retirement application. However, this has never been a real problem, given the universal understanding that performance evaluations often have little relationship to performance. Anyway, few would challenge a supervisor who refused to record the diminished performance of a once excellent, but now ailing, subordinate.