I think there were two reasons:
1. A lot of time you'll see coaches take a timeout like that to "encourage" the officials to review the play (it was inside two minutes, so PC didn't have the option of challenging). Sometimes teams will run up and get a play off before an opposing coach/the refs can review a play.
2. The clock was running when he caught the ball. If you look at the clip, you can see the clock moving; with 1:15 left, there were 38 seconds left on the play clock. Washington could've milked the clock down to ~37 seconds left before running a play. When we got the ball back at the end of the game, there were 59 seconds left on the clock. So you could argue that PC's timeout saved about 22 seconds on the clock.