When you use :: | || it is making the adjustment from that point FORWARD. This is primarily useful when your track changes tempo. If the track has a consistent tempo throughout, you typically want to use || | || as that anchors the first beat and adjusts the entire beat grid from that point.
Two tracks in sync WILL stay in sync after a tempo change due to a beat grid adjustment; try it by doubling or halving the timing on one of your tracks from a certain point. That one track will stay at the same tempo (as the master) but the OTHER track, which is sync'd will now double or halve its tempo in order to match the change from the beat grid.
Again, the need to use the :: | || is primarily because your track does NOT have a consistent tempo.