The usual name for this phenomenon in linguistics is
reduplication. One also finds it in the personal pronouns, e.g.
memet &
sese (Japanese does it also, at least with
wa ["I"] -
re [exclusive group] = 我々
ware[ware] = "we").
As to why? Because Latin is old enough to remember its Indo-European roots! Reduplication does different things in different languages, but in Latin (much as in Turkish, e.g.,
güzel 'beautiful'
> güpgüzel 'very beautiful') intensivity/emphasis is the general idea. If you have a relative adverb like
ubi 'where', then
ubiubi is even 'where-ier'!
CVR VERBA QVAEDAM LATINE DVPLICANTVR? (by the way, is it Ok?)
Bonum'st, amice mi flatulente.