Hi Krystle!
You have some excellent questions that I will try to help answer for your upcoming Walt Disney World vacation.
If you are wishing to go to the special event Magic Kingdom party,
Mickey's Not So Scary Halloween Party, you will indeed have to purchase that specific party ticket. Guests who use a day ticket for that park up to the 7pm start of the party will still need to have the party ticket to remain in the park after 7pm. The park will close to all non ticketed guests at 7pm and this means they will not be allowed to stay for attractions, shopping, or even lingering on Main Street in hopes to see the parade and fireworks. Cast members do what they call a "sweep" of the park where they let guests not wearing the wrist bands for the party know that they must exit quickly.
If you have purchased a party ticket you will be allowed to enter with only that ticket a couple of hours before the party begins which is a perk. That is to allow guests to find some dining and do some shopping early so they can just enjoy the party events once 7pm begins.
Guests may leave and re-enter a park that they hold a ticket for on that day. If guests have the Park Hopper option with their ticket they can also go to other parks on the same day. If you are planning to be in the Magic Kingdom on a party day you can come and go but once 7pm arrives you cannot enter the park again unless you have the extra party ticket.
I hope this helps clarify some of the rules concerning the special event parties and how you can enjoy a regular day as well as a special evening.