I'm in MD (next door to PA) so I do a handful.  PA has taxes for where you live, and where you work.  Many times these two jurisdictions don't talk to each other so when payments or withholdings are made based on the wrong address it becomes a bureaucratic nightmare.

If you work in PA but live elsewhere your employer should have been withholding the proper local taxes.  If so, no return is required unless there is other income (like a Sch C).  But if you live in PA and work in MD (for example) the MD employer [probably] won't be withholding local PA taxes (check the W-2 to make sure) so a return is required to pay the taxes due.