Combining whole brain and seed-based tracks

Hello experts!

I was wondering what the rationale behind combining a seed-based tractography with a whole-brain tractography is before using SIFT and SIFT2?

Intuitively, one might think that using the results of the seed-based tractography would be sufficient, but I saw the combination being done in a recent publication (Liebe et al., 2022, Mol Psy) and also described as a part of tract delineation here in the forum (i.e. Finding tracts from one region to another region and Tckedit after targeted tracking).

Thanks and bests,
Philipp