It appears that either PA is doing a spot audit of your claimed unreimbursed business expenses or considers your claimed expenses out of line for your occupation. So they are asking for a letter from your (former) employer to verify that your claimed expenses were ordinary and necessary for you to perform your job.
If you do not provide the requested information, PA will likely disallow the deductions and send you a tax bill, possibly with penalties and interest.
See for more information:
http://www.revenue.pa.gov/FormsandPublications/FormsforIndividuals/Documents/Personal%20Income%20Tax...
