Question

Does the @error line even make sense here? Would it even be hit in an error scenario? What happens if there are multiple sql statements inside a try block each followed by a similar check?

begin try
    Select a,b from tableC where a > 3 and b < 2
    Set @error = @@error
    If @error <> 0 begin raiseerror("error",16,1)end
end try
begin catch
    select error_message()
end catch

Answer 1

No, it never gets hit. This is easy to demonstrate here:

DECLARE @error int
begin try
    Select 1/0
    Set @error = @@error
    If @error <> 0 
        begin raiserror('error',16,1)
        end
end try
begin catch
    select error_message()
end catch

SELECT @error

I suspect someone updated some old code without really understanding what they were doing.

The @error will be set to 0 after each successful statement, but never get a value assigned in case of an error since it will be bypassed by the TRY...CATCH.