Removing powder paint from threads
Taps are useful for cleaning threads, and much better than forcing a screw down the hole (though that can sometime be enough).  

Derailleur mounting bolts are an unusual size, but assuming you've got a removable hanger that should be held on by a normal metric screw (M6).  Metric M5 screws are common for bottle cages, mudguards etc.

Normally you'd put a few drops of oil on the tap for actually cutting a thread.  When cleaning a thread, grease can be better as you'll pick up more of the muck with it (and leave the threads nicely greased).  If you've got a full set of taps, a second cut tap or even plug tap is enough for cleaning threads, and easier to start without cross-threading than a taper tap.