I think synchronized waits for the next monitor scan before refreshing the graphics display. On CRT monitors there could be slight flickering/banding effects if the graphics refreshed part way through a scan and the scene was changing rapidly (e.g. moving ocean waves).
Edit: Wikipedia says it all much better than I can

:
https://en.wikipedia.org/wiki/Screen_tearing
I've not seen the problem for many years. I think it's probable that modern graphics drivers do all this automatically by default, although I have no idea how they respond to ancient SH3 code. There may even be an obscure graphics driver setting that allows the "synchronized" option to do something noticeable...
I would leave synchronized on as it may save your PC from wasting cycles/watts generating frames that are never actually displayed.