VSp is the only measurement that has two indexes associated with it - depth and time. All other measurements have only one. Borehole Measurements have depth as their index and seismic has time as its index.
Therefore, in order to integrate borehole measurements with seismic the VSP data provides necessary dual index data.
T-d data with sonic measurements help generate synthetic seismogram and corridor stack provides actual zero angle reflectivity seismic response. These together help establish integration with surface seismic including identification of any spurious characteristic in surface seismic e.g. multiples.
Another most important application of VSP is its ability to look ahead which is extremely important while drilling through over pressure zones. Although inversions of VSP data are non-unique in absense of broadband data still it normally and mostly provides good indication of any significant abnormality below the bit through various inversion Schemes out of which Bayesian inversion is the best.
An excellent post on VSP technique