System Frame Number in LTE Specifications
System Frame Number (SFN) is really just a counter. As its name, it tells us the index of system frame modulo 1024, the index is from 0 to 1023 (10 bits).

SFN does not help the physical synchronization (frequency carrier, DFT starting frame, pilots, ...) because it is the information obtained after the waveform synchronization. It is decoded in Master Information Block (MIB) in Physical Broadcast channel (PBCH) as 8 bits. The SFN of MIB changes every 4 frames (2 bits) thus its value is 8+2 = 10 bits.

Nonetheless, SFN does help physical layer protocol synchronization, i.e. synchronization in term of scheduling, between DL physical channels and UL physical channels (just from release 13 for cat-M or NB-IOT UE with long repetition), or synchronization between UL physical channels (for CQI reporting, SRS scheduling, ...). For more details you can take a look at specfication 36.213 all version, the scheduling of SRS, CQI, ... all depend on $n_f$ which is SFN, i.e. absolute SFN modulo 1024.