Most cars do not have or need reverse synchros because the car is not rolling (and therefore the output shaft not turning) when you mesh the reverse gears. Here is a bad example, but the best I can think of early in the morning:
Visualize a spinning propellor, like that of an airplane or boat. That represents your transmission output shaft, and is spinning whenever the car is in motion. Now visualize another shaft, with a 2-tined pitchfork at the end. The idea is to get the propellor to engage the tines so as to drive the second shaft (the pitchfork handle in this case).
If you just shove the pitchfork into the spinning propellor, I think you can appreciate the results. You need to get the pitchfork spinning at the same rate as the propellor first, plus in proper alignment. So, how? Most aircraft propellors have a pointed center hub for aerodynamics. So let's add an extension to the pitchfork shaft with a spring-loaded matching cone. The propellor shaft center hub engages the pitchfork shaft cone just before the tines reach the spinning propellor. The hub and cone are surfaced with friction material, and act like an engaging clutch. This clutch (sychro) spins up the driven element to match the speed of the propellor, and the tines mesh with no clash of metal (grinding of gears in a transmission).
No doubt a REAL mechanic could provide a better explanation.
In the "old days" (the 50's), first gear had no synchros either. You had to come to a complete stop (output shaft not turning) to engage first without grinding your gears.