The library subscribes to databases to provide access to periodical articles. The content contained in these databases are typically not freely available on the internet. You have to pay to get access to them. But with an article database, the library is paying for that access for you. That's why you have to go through the database to get the articles.
Don't ever pay to download an article from the internet--talk to a librarian, and we can more than likely get you a copy of the article at no charge to you. That's what libraries do!