For example: if you have sales per customer and you have a customer address, customer contact person, a phone number, a visit location, a country etc. You don't want them to join to your sales. In that case for every single sales order you'll duplicate all those dimensions. So you'll get way to much data which can make Qlik slower. One link between tables does the same thing as joining the data. Only it is way more efficient, because you'll need to load the data once.
check this post for the best practices: