Vegetation Monitoring - Why permanent plots
The answer boils down to statistics rather than anything particular about this type of experiment.

If you have a sample of plots, and then take repeated measurements in that same sample over time, the variance in the values you record is a function of the variance of the time-dependent process causing changes.

If you take future measurements from a new sample of plots, you have additional variance to account for due to choosing a new sample. That is, even if you would have measured no change at all in the same plots, you expect the second sample will be different from the first just because of sampling. Because of this additional variance, you have less ability to detect smaller changes over time: less statistical power.