Why we need to use initializer list while initializing reference members?

There are lot of questions on S.O related to initializer list ,read quite a few of them ,while reading I understood why we should use intializer list in intializing const member functions,but some how i am not able to understand on why we need to use initializer list for intializing the refernce members. Consider this sample code :

class Test {   
 int &t;
 public:    
 Test(int &t):t(t) {} 
 int getT() { return t; }};
 int main() {    
 int x = 20;    
 Test t1(x);   
 cout<<t1.getT()<<endl;  
 x = 30;    
 cout<<t1.getT()<<endl;    
 return 0;
 }

can anyone please explain why we need to use this