From the WUC session on utility scripts:

rebuildLineage.pl

Q What is the purpose of the script, why would you run it?
A1 OCD people don't want gaps in lineage and run it periodically
A2 bugs that create errors in lineage, this one fixes that (especially if you have updated sites) was more a response to bugs in 6.x series.

Q Should or could I run it periodically?
A You could, but there is no reason to do that, since lineage is created when an asset is saved. 

search.pl

Q What is this script for?
A It is a response to the upgrade process that had to be run in certain upgrades cases (see gotcha.txt).

thumbnailer.pl

Q When should I run this?
A If you have changed your thumbnail size and want all your images to use this new size.

diskusage.pl

Q What exactly is beeing calculated?
A The database and file diskusage per asset.