Why would anyone purchase for such a high price at localbitcoins.com when bitcoin is being sold for less or its original
price at other websites or apps?
Localbitcoins does not act as a vendor of bitcoin. Rather, think of it as just a platform where people can list their offers for buying and selling bitcoin. These people who are listing the offers obviously want to earn from it and take advantage of the spread. They're not interested in selling their bitcoins for less than market rate, they want the opposite to be true.
The reason why you see the spreads so high on localbitcoins is that you're only limited to seeing the payment processor/country you're interested in(meaning less volume in general) and also because the payment methods are a lot more high risk and does not require ID, for example bank transfers which require ID on exchanges.
Nobody would buy high amounts and trade actively on localbitcoins. It's just a conevnient and handy way to get BTC.