The extension it granted will allow three more days to receive ballots that were mailed before polls closed. Democrats and their allies cited the prospect of postal service delays in invalidating huge numbers of ballots.
https://www.mcall.com/news/pennsylvania/mc-nws-pa-supreme-court-mail-in-ballot-20200917-qfcxnhvtsffbvkcagimmw4vaj4-story.html