The sonic is used to fix the sections of seismic reflection in depth using the sonic relation time-depth. It is used to convert the logs recorded according to depth into logs recorded according to time.
It makes it possible to calibrate with the wells the velocity model deduced from the seismic . It is used to calculate the synthetic seismograms. The synthetic seismogram simplest is the impulse seismogram which use the reflection coefficients . The reflection coefficients are calculated from the sonic log and the density log.

Example of calculation of the reflection coefficients
(IGL document).