Collected on the Internet
Many adult congregants and children also come to services in costumes depicting the Purim characters and other notable figures from Jewish history and contemporary life. According to one interpretation, this is because just as God was "hidden" throughout the Purim story, so do we "hide" behind masks and costumesAs is often the case with retro-reasons, historical accuracy is somewhat beside the point. Pleasant as it is to imagine a commitee of Rabbis, gathered around a large oak table, making decisions ("Yes! Masks! What a wonderful way to emphasize this important theological message!) the real reason for the custom is far more mundane.
The sad, unhappy truth, is the custom of donning masks and costumes on Purim was borrowed from the surrounding culture. It was first reported in Provence in the early fourteenth century, where the Jews of Italy, who observed the Italian Catholics' Lenten carnival at around the same time of year, adopted this custom for themselves; later it achieved popularity under the influence of the German Fastnacht celebration; later still it was tied to the idea of God's "hiding his face" as found in the Talmud. [Source]
So, if you think about it, wearing a mask on Purim is a little like wearing a blue shirt to shul on Shabbos - only the mask has the support of a clever little retro-reason.