I don't understand halloween. I don't know why it exists.
Some denominations of Christian churches recognize November 1 as "All Saints Day" or "All Hallows' Day", where the word hallows is derived from hallowed meaning holy. The evening prior, October 31, was "All Hallow's Eve" or shortened to "Hallow E'en", where "E'en" as a contraction means "evening".
The practice of Trick-or-Treat is as separate from a holy day...
as a team of flying reindeer pulling a sleigh with Santa Claus is from the Nativity (both celebrated as Christmas).
Here are some links about Halloween and Trick-or-Treat, which may help answer your question why this exists -
http://www.halloweenhistory.org/,
http://www.history.com/topics/halloween,
http://en.wikipedia.org/wiki/Trick-or-treating,
there are many more links, easily found through search engines.
