Parallel voltage sources...
You are right - really, there are two voltage sources which at some point are connected in parallel. One of them is imperfect with varying input voltage V1 and internal resistance R; the other is perfect with constant reference voltage 0.6 V and zero internal resistance. The input source is always connected to the output (load, not shown here). The diode acts as a switch that controls the reference source.
If V1 < 0.6 V, the diode switch is off and the reference source is disconnected. The input voltage is applied through the resistor to the output. There is no problem if there is no load connected (open circuit) - the full input voltage appears at the output. But if a significant load RL is connected, a voltage divider R-RL is formed and the output voltage decreases.
If V1 > 0.6 V, the diode switch is on and the perfect reference source is connected in parallel to the imperfect input source. The perfect source dominates over the imperfect one and its voltage appears at the output. There is no problem if a significant load is connected since the internal source resistance is low.
From these observations we can formulate a rule for parallel connection of voltage sources:
If we connect in parallel two voltage sources - perfect and imperfect, the voltage across this network is equal to the voltage of the perfect source.
The advantage of this clever trick is that two voltage sources can be switched to the output by a simple 2-terminal switch (SPST) instead by the more sophisticated 3-terminal switch (SPDT). SPST can be implemented by a diode.
What is the role of R?
At first glance, the resistor R is redundant and only worsens the circuit operation... but in fact its role is extremely important. By it the input source is "deliberately worsen" to prevent the conflict between two perfect voltage sources.
With the same success, we can swap the source's properties making the input source perfect and the reference source imperfect. This means to swap the resistor and diode. Thus we will obtain another kind of diode limiters.
In total, there are eight variations of this device known as a "diode limiter".