Cyclic prefix vs. guard interval. Why use one over the other?
A silent  guard interval would likely increase PAPR (peak to average power), which requires a sender capable of more dynamic range (without clipping or generating power supply noise, etc.)

A long enough cyclic prefix prevents multipath echoes from one symbol interfering with the portion of the next symbol after the prefix.  And a symbol interfering with itself from only its own multipath echo can be modeled as a flat fading channel, which may be easier to equalize or to error correct over a far wider range of multipath channel responses.