Timing over Packet (ToP) began as a sideshow of the Circuit Emulation wave that flooded the telecom industry in the early 2000s. Reinforced by the emerging new PWE3 family of pseudowire (PW) standards in the IETF, more and more carriers began looking for cheaper ways to carry their legacy TDM services, giving up traditional expensive leased lines.
Back in those days, ToP was merely considered a way to enable timing "transparency" across a physical layer asynchronous network (today called Adaptive Clock Recovery or ACR). The term "transparency' is placed in quotation marks, as the performance of early ToP mechanisms fell short of what was needed to ensure true end-to-end clock transparency.
This has radically changed today as modern ToP mechanisms are rapidly approaching the performance envelope of their SDH/SONET counterparts, enabling conformance to the latter's most stringent timing requirements.