1) In most congregations, the Ba'al Tefilah (one leading the services) for Selichos wears a Tallis, even if the Selichos is being said before daybreak (or at night after Chatzos, Halachic) which is before the proper time for Tzitzis and Tallis.
It is best for the Ba'al Tefilah to use the Shul's Tallis or a friend's, but not his own, as to not get into the problem of perhaps needing to recite a Bracha (as some Poskim maintain that a Bracha is required even at night on your own Tallis.)
2) The reason it is so important for a Tallis to be worn by Selichos, is as follows:
The Talmud (Rosh Hashana 17b) relates that Hashem Himself in all His glory wrapped Himself in a Tallis like a Ba'al Tefilah and appeared to Moshe Rabeinu and showed him the appropriate way for Klal Yisroel to pray (and thus merit the forgiveness of Hashem).
It was then that Hashem revealed to Moshe the "Yud Gimel Middos Shel Rachamim" (Tefilah containing the 13 attributes of mercy of Hashem), which we will say countless times between now and Yom Kippur, and is perhaps the most exalted and powerful prayer known to mankind. (, we shall discuss the 13 Middos more at length)