77s (these days) align on double-word boundaries. SYNC cannot not change that.
01s align on double-word boundaries. Their subordinate items, however, are not aligned. Unless a binary value and you supply SYNC. In which case you may get "slack bytes" - "gaps".
Unless the documentation of a product you are using specifies that you must
use SYNC, don't use it. It is a throwback to distant times.
In those distant times, at least some "halfword" machine-instructions required that the source field started on a halfword boundary and would abend if given an address which was not on a halfword boundary. The COBOL compiler at the time, knowing this, would generate a move to a piece of compiler storage aligned correctly, do the operation, then move it back. The extra code generated could be avoided by specifying SYNC, at the cost of one byte of working-storage.
I have never seen a compiler generate that code. It is a long time since the machine-code itself, the instructions, were changed so that they did not require boundary-alignment.
Forget all about using SYNC, unless a document for something you are using says otherwise. A document. Not "someone" who once heard a rumour and never bothered to check on it. See it in a document