Link tables are generally used for linking the two table or the fact tables.
Let us go with an example..
As we know that when we are designing the datamodel synthetic keys and circular loop are common.
For fixing this problems we use the link table concept.
You can also use concatenation ,but it always not give the appropiate result.
Concatenation works well enough when we have all the fields same.
With the use of link tables, it’s possible to keep the facttables separated from each other. The advantage of this solution for choosing this method is to keep the datamodel a logical one.
see the below link it helps u more