I think it is because halloween is the night of evil and so the witches and warlocks cast spells to get rid of the evil.
Tomorrow is all saints day, quite the opposite of halloween and evil.

Simple and fun explanations;

http://www.brownielocks.com/halloweenhistory.html

I don't remember halloween at all from my childhood only bonfire night . I thought it was recent import from the USA .

There was always Allhallows eve but that was not a commercialised occasion.

All Hallows' Eve and All Saints' Day were, from what I understood religious days, so I thought, as Meg did, that Halloween was a comparatively recent import from the US.

And I do not understand where the habit of "trick or treat" came in.

All hallow's eve is the night in which the barrier between the realm of the living and the realm of the death is only thin like a curtain. If you want to communicate with someone who's passed away, halloween is the night to do it.
Also, the word 'witch' in German is 'Hexe' which comes from 'Hagazussa', which means 'sitting on the fence between realms'. According to legend witches and wizards were able to wander between the two worlds. But on halloween, since the barrier is so thin, normal people were able to as well. Perhaps they dressed up as witches to fool the barrier, or maybe it's just a bit of fun.

There's a lot of interesting history about Halloween and the origins of it. I don't celebrate the commercial part of it but I do think of Samhain on the day. The 1st of november is the first day of the new year (in the northern hemisphere) of the celtic calendar.

Very interesting Florence.

And a great link there Nippy! Admit I did laugh when I got to the bit called
WHAT ABOUT THOSE GOBLINS?

Never trick or treated in my life and have never been trick or treated in my lifetime. Don't know what date its on

Hallowe'en isn't a US import. It used to be Myschief Night on the 29th October in the UK & then All Hallows Eve.

It means s*d all to me

lol! Same here. I'm intrigued to see that on Cyprus local forums some Brits are desperately asking where they can buy fancy dress for Halloween! Now as most Brits who come here are retired or at least getting on a bit I do wonder why they miss it so much they need to 'recreate it'.

But, it's probably the same as St Patricks Day - none so Irish as the Irish (or even any Brit) as those who've left the UK! They go loopy in the pubs and for me it is definitely a day and night to stay off the streets! I've never celebrated it and I'm a Murphy!

Begging in an intimidating way