A database is used to store data for later usage. Users of your site might want to have a look in parts of your database. This way, in case of a webshop, the user can check his order history or, in case you run a database which keeps track of working hours, your employees could have a look at the hours worked during the last month.
Linking your databases makes your website more interactive with the user and sharing information with clients becomes more easy. Oqapi can link all kinds of databases with one another such that particular information is sent to the right users