Why is the value of A1 A2 set as 0xF628 in SONET frames?

The hex code F628 is transmitted in every frame of every STS-1.

This allows a receiver to locate the alignment of the 125usec frame within the received serial bit stream.  Initially, the receiver scans the serial stream for the code F628.  Once it is detected, the receiver watches to verify that the pattern repeats after exactly 810 STS-1 bytes, after which it can declare the frame found. Once the frame alignment is found, the remaining signal can be descrambled and the various overhead bytes extracted and processed.

Just prior to transmission, the entire SONET signal, with the exception of the framing bytes and the section trace byte, is scrambled.  Scrambling randomizes the bit stream is order to provide sufficient 0-1 and 1-0 transitions for the receiver to derive a clock with which to receive the digital information.

Scrambling is performed by XORing the data signal with a pseudo-random bit sequence generated by the scrambler polynomial indicated above.  The scrambler is frame synchronous, which means that it starts every frame in the same state.

Descrambling is performed by the receiver by XORing the received signal with the same pseudo random bit sequence.  Note that since the scrambler is frame synchronous, the receiver must have found the frame alignment before the signal can be descrambled.  That is why the frame bytes are not scrambled.

One more interesting answer from Standardisation Expert ITU-T Huub van Helvoort:-

The initial “F” is to provide four consecutive bits for the clock recovery circuit to lock the clock. The number of “0” and “1”in F628 is equal (=8) to take care that there is a DC balance in the signal (after line coding).