Frame acquisition vs symbol timing synchronization
Auto-correlation will be enough to determine the symbol center of 1 symbol at the end of the preamble, so that is sufficient for acquisition.

Symbol center tracking throughout the frame is still required to keep the receiver symbol clock synchronized to the transmitter's symbol clock.  Your receiver's symbol clock frequency will never be exactly the same as the transmitter's symbol clock frequency in real implementations, therefore continual estimation and correction or tracking is needed.