MK closes at 7:00 p.m. unless you have a party ticket. With a party ticket you will be issued a wristband (when you are already in the park on a regular ticket- there are stations where you take your party ticket to get your wristband). At 7:00 p.m., CMs are checking for wristbands. If you do not have one, you are blocked from going deeper into the park, directed to the exit, and are not allowed on the rides, into the stores, or often even into the bathrooms (we personally observed a Mom and child get refused bathroom access).
If you do not have a regular park ticket or if you do not have a hopper and have already used that days ticket in a different park, then you can use your party ticket to get admission into MK 3 hours before the party officially starts ( I believe the point of that is to stagger people entering the park, so you don't have everyone attempting to enter at the same time at 7:00 p.m.). The official party runs from park close at 7:00 p.m. to midnight.
So, your regular ticket will not get you party access. Even if you spend the full day in MK on your regular ticket, unless you buy party tickets you will have to leave at 7:00 p.m. For us the party tickets are worth it- we really enjoy the Halloween parade, castle show, party characters, Haunted Mansion area, and magic photo shots. We could care less about the trick or treating. We aren't going to cart all the candy home with us in our luggage on the plane, and we get enough Mickey head ice cream bars and other specialty desserts at WDW that we don't need ordinary candy. The lines can be long earlier in the night, but later more towards the party end they are virtually non-existent. DD usually goes through some of the lines just for fun late when they are walk through. They have never been out of candy when she has done that. Even just doing that, there is always candy left behind in our hotel room when we leave.