Concatenate is used when you need to merge two or more tables that have identical structures. In the example you have explained, other than patient identifier there is no other columns identical then it is better to keep the tables separate linked to patient identifier.
Check out this document. It will help you understand the differenct between the Join and Concatenate functions. I would like to give credit to the person who created it, but I don't remember where I got it.
I don't believe your statment is correct. Here is a description of Concatenate from the help text:
A concatenation is in principle the same as the SQL UNION statement, but with two differences: first that Concatenate prefix can be used no matter if the tables have identical field names or not; and secondly that no removal of identical records are made.
Though you can force concatenate the tables, I meant to say Concatenate statement could be recommended where you need to merge two or more tables that have identical structures and pertaining to the situation.