PTP is a good choice for synchronizing clocks on a per-segment basis, but it doesn't work across a WAN and there are lots and lots of devices out there that don't support it.

NTP is good for doing time sync across the WAN, but to get the most out of it for certain applications you need to hook it up to reference clocks.

So, NTP can use PTP as a reference clock, and Bob's your uncle.